Abstract:
A method of forming a near field transducer (NFT) for energy assisted magnetic recording is disclosed. A structure comprising an NFT metal layer and a first hardmask layer over the NFT metal layer is provided A first patterned hardmask is formed from the first hardmask layer, the first patterned hardmask disposed over a disk section and a pin section of the NFT to be formed. An etch process is performed on the NFT metal layer via the first patterned hardmask, the etch process forming the NFT having the disk section and the pin section.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to magnetic recording heads and, in particular, relates to double hard-mask mill back methods of fabricating a near field transducer for energy assisted magnetic recording. 
     BACKGROUND 
     To increase the areal storage density of a magnetic recording device, the recording layer thereof may be provided with smaller and smaller individual magnetic grains. This reduction in grain size soon reaches a “superparamagnetic limit,” at which point the magnetic grains become thermally unstable and incapable of maintaining their magnetization. The thermal stability of the magnetic grains can be increased by increasing the magnetic anisotropy thereof (e.g., by utilizing materials with higher anisotropic constants). Increasing the magnetic anisotropy of the magnetic grains, however, increases their coercivity and therefore requires a stronger magnetic field to change the magnetic orientation of the grains (e.g., in a write operation). 
     Energy-assisted magnetic recording (EAMR) is used to address this challenge. In an EAMR system, a small spot less than ¼λ where data is to be written is locally heated to reduce the coercivity of the magnetic grains therein for the duration of the write operation, thereby allowing materials with increased magnetic anisotropy to be used, and greater areal storage density to be exploited. 
     In EAMR approach, a semiconductor laser diode is normally used as a light source and coupled to a planar waveguide which serves as light delivery path. A grating structure may be used to couple the laser light into the waveguide. The coupled light is then routed to a near field transducer (NFT) by which the optical energy is provided to a small optical spot on the recording media a few tens of nanometers (nm) in size. 
       FIG. 1  is a diagram depicting a perspective view of a so-called “Puccini-type” NFT″  100  comprising a narrow pin section  132  connected to a small disk section  134 .  FIG. 2  is a diagram depicting a cross-sectional view of an NFT arrangement  200  in which the NFT  100  is coupled to a waveguide structure  210  via an NFT writer gap layer  220 . The pin section  132  has pin length  133 , and the disk section  134  has disk size (e.g., diameter)  135 , and the NFT  100  has NFT thickness  131 . The NFT writer gap layer  220  provides writer gap  221  between the waveguide structure  210  and the NFT  100 . In the illustrated example, the waveguide structure  220  is a waveguide core layer. Traditional approach for fabricating an NFT arrangement such as  200  of  FIG. 2  involves using a milling process to form the pin section  132  followed by a lift-off process to form the disk section  134 . For example, the pin section  132  is formed first by ion milling. Second, photolithography is used to form a hole which is aligned with the already-formed pin section  132 . Third, the hole is filled with a metal (e.g., Au). Finally, the pin section  134  is formed from the filled metal following a lift-off process. 
     The traditional fabrication approach and a final NFT structure fabricated thereby suffers from a number of limitations. The writer gap  221  cannot be controlled accurately because during pin milling process, partially-exposed material of the writer gap layer  220  is milled away. In addition, during second NFT photolithography, partially-exposed material of the writer gap layer  220  is also etched away by developer. The writer gap  221  variation depending on over-milling time and photo-rework frequency. The NFT thickness  131  cannot be controlled accurately due to shadow effect and lift off milling process. The NFT thickness  131  variation depends on disk size and photo thickness. The disk size  135  has a lower limit because, with current techniques, a hole formed by the photolithography is limited to a diameter larger than 250 nm. The NFT shape is not consistent since fencing- and bow-shaped surface is typical result of a lift off process. 
     Accordingly, there is a need for NFT fabrication methods that address the aforementioned limitations associated with the traditional NFT fabrication approach. 
     BRIEF SUMMARY OF THE INVENTION 
     In certain aspects, a method of forming a near field transducer (NFT) for energy assisted magnetic recording is disclosed. The method comprises providing a structure comprising an NFT metal layer and a first hardmask layer over the NFT metal layer. The method can further comprise forming a first patterned hardmask from the first hardmask layer, the first patterned hardmask disposed over a disk section and a pin section of the NFT to be formed. The method can further comprise performing an etch process on the NFT metal layer via the first patterned hardmask, the etch process forming the NFT having the disk section and the pin section. 
     In certain aspects, a method of forming a near field transducer (NFT) for energy assisted magnetic recording is disclosed. The method comprises providing a structure comprising an NFT metal layer and a hardmask layer over the NFT metal layer. The method can further comprise forming a patterned hardmask from the hardmask layer, the patterned hardmask disposed over at least a disk section of the NFT to be formed. The method can further comprise removing a portion of an exposed region of the NFT metal layer not covered by the patterned hardmask, thereby forming at least the disk section of the NFT. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a diagram depicting a perspective view of an exemplary Puccini-type NFT. 
         FIG. 2  is a diagram depicting a cross-sectional view of an NFT arrangement in which the NFT of  FIG. 1  is coupled to a waveguide structure via an NFT writer gap layer. 
         FIG. 3  is a flowchart illustrating an exemplary process for fabricating an NFT according to certain aspects of the subject disclosure. 
         FIGS. 4A-4G  depict structures arrived before, during, or after various operations of the process of  FIG. 3  according to certain aspects of the subject disclosure. 
         FIG. 5  is a flowchart illustrating an exemplary process for fabricating a first patterned HM to be used for forming (e.g., milling) an NFT structure according to such alternative embodiments of the subject disclosure. 
         FIGS. 6A-6E  depict structures arrived before, during, or after various operations of the process of  FIG. 5  according to certain aspects of the subject disclosure. 
         FIG. 7  is a diagram depicting a perspective view of an exemplary Puccini-type NFT in which pin section and disk section have different thicknesses according to certain aspects of the subject disclosure. 
         FIG. 8  is a diagram depicting a cross-sectional view of an NFT arrangement in which the NFT of  FIG. 7  is coupled to a waveguide structure via an NFT writer gap layer according to certain aspects of the subject disclosure. 
         FIGS. 9A-9D  are diagrams illustrating an exemplary two-step mill back method for fabricating an NFT having different disk and pin thicknesses according to certain aspects of the subject disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a flowchart illustrating an exemplary process  300  for fabricating an NFT (e.g.,  100  of  FIG. 1 ) according to certain aspects of the subject disclosure.  FIGS. 4A-4G  depict structures arrived before, during, or after various operations of the process  300  of  FIG. 3  according to certain aspects of the subject disclosure. Each of  FIGS. 4A-4G  provides a cross-sectional view (e.g.,  401 A) and a top view (e.g.,  401 B) of a corresponding structure. For ease of illustration, without any intent to limit the scope of the subject disclosure in any way, the process  300  will be described with reference to the structures  401 A- 407 A,  401 B- 407 B depicted in  FIGS. 4A-4G . 
     The process  300  begins at start state  301  and proceeds to operation  310 , in which a structure, whose cross-sectional view  401 A and top view  401 B are depicted in  FIG. 4A , is provided. As seen from the cross-sectional view  401 A, the structure comprises a waveguide layer  410 , an NFT writer gap layer  420  over the waveguide layer  410 , an NFT metal layer  430  over the NFT writer gap layer  420 , an etch stop (ES) layer  440  over the NFT metal layer  430 , a first hardmask (HM) layer  450  over the ES layer  440 , a second HM layer  470  over the first hardmask (HM) layer  450 , and a third HM layer  480  over the second HM layer  470 . 
     The waveguide layer  410  can include any transparent or semi-transparent material including, but not limited to, TiO 2 , Ta 2 O 5 , Si, SiN, and ZnS, and can have a thickness in a range between about 1,200 and 2,000 Å. The NFT writer gap layer  420  can include, but are not limited to, an optical-grade ALD alumina, and can have a thickness in a range between about 100 and 200 Å. The NFT metal layer  430  can include any metal capable of supporting a surface-plasmon resonance (SPR) therein including, but not limited to Au, Ag, Al. and a combination thereof. The NFT metal layer  430  can have a thickness in a range between about 300 and 500 Å. The first HM layer  450  can include a material (e.g., dielectric) selected from the group consisting of SiC, amorphous carbon, and diamond-like carbon (DLC), and can have a thickness in a range between about 300 and 1,000 Å. In the illustrated example, the first HM layer  450  includes SiC and has a thickness of about 500 Å. The second HM layer  470  can include a material selected from the group consisting of Cr or CrN, and can have a thickness in a range between about 30 and 100 Å. In the illustrated example, the second HM layer  470  includes Cr and has a thickness of about 50 Å. The third HM layer  480  can include a material selected from the group consisting of Ta and Ta 2 O 5 , and can have a thickness in a range between about 30 and 100 Å. In the illustrated example, the third HM layer  480  includes Cr and has a thickness of about 50 Å. 
     The ES layer  440  can include any material that is resistant to the etch process used for etching the first HM layer  450  including, but not limited to, Cr, Ru, and CrN. In the illustrated example in which the first HM layer  450  comprises 500 Å-thick SiC, the ES layer  440  includes 20 Å-thick Cr. The second HM layer  470  also functions as an etch stop layer with respect to the etching of the third HM layer  480 . In the illustrated example in which the third HM layer  480  comprises 50 Å-thick Ta, the second HM layer  470  is a 50 Å-thick Cr layer that is substantially resistant (e.g., etch selectivity of greater than 20:1) to the etch process (e.g., Ta RIE) used to etch the Ta layer  480 . 
     The process  300  proceeds to operation  320 , in which a third patterned HM  482  ( FIG. 4B ) is formed from the third HM layer  480 . The third patterned HM  482  includes a shape of pin section  332  ( FIG. 4G ) of the NFT to be formed. In the illustrated example of  FIG. 4B , the operation  320  includes forming a first patterned photoresist (PR # 1 )  492  over the third HM layer  480  and performing a first etch process (e.g., Ta RIE) to remove portions of the third HM layer  480  not covered by PR # 1   492 . The PR # 1   492  can be formed by a suitable standard photolithography. The second HM layer  470  (e.g., Cr) functions as an etch stop for the first etch process. It shall be appreciated that the Ta—Cr combination for the third and second HM layers  480 ,  470  are exemplary only, and a multitude of other combinations including, but not limited to, Ta—CrN, Ta 2 O 5 —Cr, Ta 2 O 5 —Cr, Cr—Ta, Cr—Ta 2 O 5 , CrN—Ta, CrN—Ta 2 O 5  may be employed. 
     The process  300  proceeds to operation  330 , in which a second patterned HM  471  having a first HM portion  472  and a second HM portion  474  is formed from the second HM layer  470  as illustrated by  FIGS. 4C and 4D . In the illustrated examples of  FIGS. 4C and 4D , the operation  330  includes forming a second patterned photoresist (PR # 2 )  494  over the second HM layer  470  and performing a second etch process (e.g., Cr RIE) to remove portions of the second HM layer  470  not covered by PR # 2   494  and the third patterned HM  482 . The PR # 2   494  includes a shape of the disk section  434  of the NFT being formed. The PR # 2   494  can be formed by a suitable standard photolithography. Because the PR # 2   492  is a disk, not a hole, disk size (e.g.,  135  of  FIG. 1 ) can be made (e.g., printed) to be smaller than 200 nm. The disk size can get even smaller by the use of an O 2  plasma trim process. In the illustrated example of  FIG. 4C , the PR # 2   494  overlaps a portion of the third patterned HM  482 . The overlapping helps to achieve a sharp corner between the pin and disk sections of the NFT to be formed. The first HM layer  450  (e.g., SiC) functions as an etch stop for the second etch process. In some alternative embodiments, an additional etch stop layer (e.g., Ta) may be inserted between the first HM layer  450  and the second HM layer  470  to stop the second etch process. 
     The process  300  proceeds to operation  340 , in which a first patterned HM  451  having a first HM portion  452  and a second HM portion  454  is formed from the first HM layer  450  as illustrated by  FIG. 4E . The first HM portion  452  of the first patterned HM  451  is disposed over the pin section  432  of the NFT to be formed, and the fourth HM portion  454  of the first patterned HM  451  is disposed over the disk section  434  of the NFT to be formed. The operation  340  includes performing a third etch process (e.g., CF 4  or SF 6  plasma RIE) to remove portions of the first HM layer  450  not covered by the second patterned HM  471 . The ES layer  440  functions as an etch stop for the third etch process and protects the NFT metal layer  430  therefrom. 
     The process  300  proceeds to operation  350 , in which a final NTE structure  431  having the pin section  432  and the disk section  434  is formed from the NFT metal layer  430  as illustrated by  FIG. 4F . In the illustrated example of  FIG. 4F , the operation  350  includes performing a fourth etch process (e.g., milling operation) to remove portions of the second ES layer  440  and the NFT metal layer  430  not covered by the first patterned HM  451 . In the illustrated example of  FIG. 4F , the NFT writer gap layer  420  (e.g., alumina) functions as an etch stop for the fourth etch process for milling of the NFT metal layer (e.g., Au). In some embodiments, the second HM  471  and an upper portion of the first HM  451  are removed during the fourth etch process. 
     The process  300  proceeds to operation  360 , in which the first patterned HM  451  is removed by, e.g., RIE process, leaving behind the structure whose cross-sectional view  407 A and top view  407 B are shown in  FIG. 4G . Optionally, if there is an optical performance concern, residual portions  442 ,  444  of the second etch layer  440  can also be removed at the same time as or separately from the removal of the first patterned HM  451 , e.g., by a wet etch process. 
     In the process  300  illustrated by FIGS.  3  and  4 A-G, fabrication of the first patterned HM  451  involves forming the third patterned HM  482  having a shape of the pin section  432  followed by forming the second patterned HM  471  having a shape of the disk section  444 . In certain alternative fabrication embodiments, this order can be reversed. For example, fabrication of the first patterned HM  451  can involve forming a third patterned HM having a shape of a pin section of the NFT to be formed followed by forming a second patterned HM having a shape of a disk section of the NFT to be formed. 
       FIG. 5  is a flowchart illustrating an exemplary process  500  for fabricating a first patterned HM (e.g.,  451 ′ of  FIG. 6E ) to be used for forming (e.g., milling) an NFT structure according to such alternative embodiments of the subject disclosure.  FIGS. 6A-6E  depict structures arrived before, during, or after various operations of the process  500  of  FIG. 5  according to certain aspects of the subject disclosure. For ease of illustration, without any intent to limit the scope of the subject disclosure in any way, the process  500  will be described with reference to the structures depicted in  FIGS. 6A-6E . 
     The process  500  begins at start state and proceeds to operation  510  in which a structure comprising a waveguide layer  410 ′, an NFT writer gap layer  420 ′ over the waveguide layer  410 ′, an NFT metal layer  430 ′ over the NFT writer gap layer  420 ′, an ES layer  440 ′ over the NFT metal layer  430 ′, a first hardmask (HM) layer  450 ′ over the first ES layer  440 ′, a second HM layer  470 ′ over the first hardmask (HM) layer  450 ′, and a third HM layer  480 ′ over the second HM layer  470 ′ is provided. The layers  410 ′,  420 ′,  430 ′,  440 ′,  450 ′,  470 ′, and  480 ′ are substantially similar to the corresponding layers  410 ,  420 ,  430 ,  440 ,  450 ,  470 , and  480  of  FIG. 4A , and are not repeated here for the sake of brevity. 
     The process  500  proceeds to operation  520 , in which a third patterned HM  484 ′ ( FIG. 6B ) is formed from the third HM layer  480 ′. The third patterned HM  484  includes a shape of disk section of the NFT to be formed (e.g., milled) via the first patterned HM  451 ′ being fabricated. In the illustrated example of  FIG. 6B , the operation  520  includes forming a first patterned photoresist (PR # 1 )  494  over the third HM layer  480 ′ and performing a first etch process (e.g., Ta RIE) to remove portions of the third HM layer  480 ′ not covered by PR # 1   494 . The second HM layer  470 ′ (e.g., Cr) functions as an etch stop for the first etch process. 
     The process  500  proceeds to operation  530 , in which a second patterned HM  471 ′ having a first HM portion  472 ′ and a second HM portion  474 ′ is formed from the second HM layer  470 ′ as illustrated by  FIGS. 6C and 6D  The second HM portion  472 ′ is disposed over pin section of the NFT to be formed. In the illustrated examples of  FIGS. 6C and 6D , the operation  530  includes forming a second patterned photoresist (PR # 2 )  498  over the second HM layer  470 ′ and performing a second etch process (e.g., Cr RIE) to remove portions of the second HM layer  470 ′ not covered by PR # 2   498  and the third patterned HM  484 ′. In the illustrated example of  FIG. 4C , the PR # 2   498  overlaps a portion of the third patterned HM  484 ′. The first HM layer  450 ′ (e.g., SiC) functions as an etch stop for the second etch process. 
     The process  500  proceeds to operation  540 , in which a first patterned HM  451 ′ having a first HM portion  452 ′ and a second HM portion  454 ′ is formed from the first HM layer  450 ′ as illustrated by  FIG. 6E . The first HM portion  452 ′ of the first patterned HM  451 ′ is disposed over the pin section of the NFT to be formed, and the fourth HM portion  454 ′ of the first patterned HM  451 ′ is disposed over the disk section of the NFT to be formed. In the illustrated example of  FIG. 6E , the operation  540  includes performing a third etch process (e.g., SiC RIE) to remove portions of the first HM layer  450 ′ not covered by the second patterned HM  471 ′. The ES layer  440 ′ functions as an etch stop for the third etch process and protects the NFT metal layer  430 ′ therefrom. 
     In the NFT structure  100  shown in  FIGS. 1 and 2  and various related NFT fabrication processes described thereafter, the pin section  132  and the disk section  134  are assumed to have the same NFT thickness  131 . However, in certain embodiments, disk section  134 ″ and pin section  132 ″ of NFT  100 ″ have different thicknesses as shown in embodiments depicted in  FIGS. 7 and 8 . In the illustrated examples of  FIGS. 7 and 8 , the disk section  134 ″ has disk thickness  131 ″, and the pin section  132 ″ has pin thickness  137 ″. The pin section  132 ″ has pin length  133 ″, and the disk portion  134 ″ has disk size (e.g., diameter)  135 ″. As with the NFT arrangement  200  of  FIG. 2 , the NFT  100 ″ can be coupled to waveguide  210 ″ via an NFT writer gap layer  220 ″. The NFT writer gap layer  220 ″ provides writer gap  221 ″ between the waveguide structure  210 ″ and the NFT  100 ″. In some embodiments, the pin thickness  137 ″ can be in a range of between about 20 and 50 nm, and the disk thickness  131 ″ can be in a range of between about 30 and 150 nm. In one exemplary NFT arrangement, the pin thickness  137 ″ is 25 nm, the disk thickness  131 ″ is 50 nm, the writer gap  220  is 20 nm, and the disk size  135 ′ is 200 nm. The pin length  133 ″ can be in a range of between about 0 and 50 nm. The pin section  132 ″ can have a width  139 ″ in a range of between about 15 and 50 nm. 
     An exemplary two-step mill back method of fabricating an NFT  100 ″ having different disk and pin thicknesses  131 ″,  137 ″ is now described. The two-step mill back method involves first forming a patterned SiC HM  154 ″ over a region of NFT metal layer  130 ″ corresponding to disk section  134 ″ of the NFT  100 ″ to be formed as illustrated by an intermediate structure  900 A depicted in  FIG. 9A . The intermediate structure  900 A further comprises a waveguide core layer  110 ″, an NFT writer gap layer  120 ″ over the waveguide layer  110 ″, and an etch stop layer  140 ″ over the NFT metal layer  130 ″. The patterned SiC HM  154 ″ may be formed, for example, from a SiC layer deposited coextensively over the underlying layers  110 ″- 140 ″ by an etch process (e.g., RIE) performed via a patterned photoresist accompanied by a photo O 2  plasma trimming. The etch process is terminated at the etch stop layer  140 ″. 
     Subsequently, the disk section  134 ″ is formed from the NFT metal layer  130 ″, e.g., by performing a first milling (e.g., ion-milling) process preformed on the NFT metal layer via the patterned SiC HM  154 ″ to remove portions in a lateral direction (e.g., x-direction) of the NFT metal layer not covered by the patterned SiC HM, 154 ″. As illustrated by an intermediate structure  900 B depicted in  FIG. 9B , the first milling operation only partially removes material of the NFT metal layer  130 ″ in the thickness direction (e.g., z-direction) of the layer  130 ″ to form the thinner pin section  132 ″. The partial thickness-direction removal of the NFT metal layer  130 ″ is achieved by, e.g., controlling an end point of the first milling operation (e.g., ending the first milling operation after a time duration X that is known to remove a thickness Y). After the first milling operation is performed on the NFT metal layer  130 ″, the disk section  134 ″, the pin section  132 ″, and residual NFT metal portion  136 ″ remain of the NFT metal layer  130 ″ as illustrated by the intermediate structure  900 B of  FIG. 9B . 
     After the first ion-milling process, a pin mask  194 ″ (e.g., a PR or hardmask) having a shape of the pin section  132 ″ is formed over an exposed region (e.g., the region not covered by the disk section  154 ′) of the NFT metal layer and stitched with the previously-formed disk section  154 ′. In some embodiments, the pin mask  194 ″ overlaps a portion of the disk section  154 ″ as illustrated by intermediate structure  900 C depicted in  FIG. 9C . Subsequently, the residual NFT metal portion  136 ″ is removed by a second milling (e.g., ion-milling) process performed via the pin mask  194 ″ and the disk section  154 ″. The pin mask  194 ″ and the disk section  154 ″ (and optionally residual etch stop  144 ″) are subsequently removed as illustrated by structure  900 D depicted in  FIG. 9D . 
     Those skilled in the art shall appreciate that various NFT fabrication methodologies of subject disclosure provide a number of advantages including the following:
         1) NFT writer gap (e.g.,  220 ) can be controlled accurately because the writer gap is defined by deposition of writer gap layer (e.g.,  420 ,  120 ″) and subsequent milling and photolithography do not affect the writer gap layer.   2) NFT disk thickness (e.g.,  131 ,  131 ″) can be controlled accurately because the NFT disk thickness is defined by deposition of NFT metal layer (e.g.,  430 ,  130 ″). Furthermore, in the case of an NFT (e.g.,  100 ″) having different disk and pin thicknesses ( 131 ″,  137 ″), the NFT pin thickness (e.g.,  137 ″) can be controlled by using an end-point controlled milling process.   3) Disk size (e.g.,  135 ,  135 ″) is extendable because there is no process limitation for the disk size.   4) Disk shape is well defined due to a large etch selectivity between the third HM layer ( 480 ,  180 ″) and second HM layer ( 470 ,  170 ″).       

     The description of the invention is provided to enable any person skilled in the art to practice the various embodiments described herein. While the present invention has been particularly described with reference to the various figures and embodiments, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention. 
     There may be many other ways to implement the invention. Various functions and elements described herein may be partitioned differently from those shown without departing from the spirit and scope of the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other embodiments. Thus, many changes and modifications may be made to the invention, by one having ordinary skill in the art, without departing from the spirit and scope of the invention. 
     A reference to an element in the singular is not intended to mean “one and only one” unless specifically stated, but rather “one or more.” The term “some” refers to one or more. Underlined and/or italicized headings and subheadings are used for convenience only, do not limit the invention, and are not referred to in connection with the interpretation of the description of the invention. All structural and functional equivalents to the elements of the various embodiments of the invention 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 intended to be encompassed by the invention. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the above description.