Abstract:
Methods for reducing feature sizes of devices such as electromagnetic sensors are disclosed. A track width of a MR sensor is defined by a mask having an upper layer with a reduced width and a lower layer with a further reduced width. Instead of or in addition to being supported by the lower layer in the area defining the sensor, the upper layer is supported by the lower layer in areas that do not define the sensor width. In some embodiments the upper layer forms a bridge mask, supported at its ends by the lower layer, and the lower layer is completely removed over an area that will become a sensor. Also disclosed is a mask having more than two layers, with a bottom layer completely removed over the sensor area, and a middle layer undercut relative to a top layer.

Description:
BACKGROUND 
     The present invention relates to devices, such as magnetoresistive (MR) sensors or electronic circuits, having submicron features that are manufactured with a mask that is undercut, with the undercut allowing the mask and overlying materials to be lifted off. 
     FIG. 1 shows a prior art step in the formation of a conventional MR sensor for a hard disk drive. Over a wafer substrate  20  a magnetic shield layer  22  has been formed, either directly on the substrate or on an intermediate layer, not shown. Atop the shield layer  22  a first read gap layer  24  of dielectric materials has been formed, and atop the read gap layer  24  a plurality of MR sensor layers  26  has been formed. A bi-layer mask  25  has been formed of layers  27  and  28 , and after photolithographic patterning, layer  27  has been chemically removed relative to layer  28 , forming undercut edges  30  and  33 . A directional removal step such as ion beam etching (IBE) has been performed to create edges  35  and  36  of the sensor layers  26 , the IBE also removing part of the read gap layer  24 . 
     In FIG. 2 a bias layer  40  has been sputter deposited, followed by an electrically conductive lead layer  44 . The electrically conductive bias layer  40  and lead layer  44  abut the edges  35  and  36  of the sensor layers  26  to stabilize magnetic domains of the sensor layers and provide electric current to the sensor layers. The bias layer  40  and lead layer  44  are also deposited atop mask layer  25 , but due to undercuts  30  and  33 , a chemical etch can be applied that dissolves mask layer  27  allows the mask and the layers  40  and  44  atop the mask to be lifted off. 
     FIG. 3 shows a cross-sectional view of the sensor layers  26 , bias layer  40  and lead layer  44  after the mask has been lifted off. This cross-sectional view of the sensor layers is essentially that which will be seen from a media such as a disk, after the wafer  20  has been diced and the die or head containing the sensor layers  26  has been positioned adjacent the media in a drive system. An active width or track width TW 0  of the sensor layers  26  between lead layers  44  may be in a range between one-half micron and one micron, corresponding to a resolution at which the sensor layers can read magnetic tracks in the media. 
     FIG. 4 is a top view of the sensor layers  26 , bias layer  40  and lead layer  44  of FIG.  3 . The wafer and thin film layers will, as mentioned above, be diced along the dashed line  3 — 3  that indicates the cross-sectional view of FIG.  3 . The sensor layers  26  shown in FIG. 4 have been trimmed along back edges  50  and  52  distal to the dashed line  3 — 3  by conventional masking and IBE such as ion milling, not shown. The leads  44  are typically so much thicker than the sensor layers  26  that the ion milling of the back edges  50  and  52  of the sensor layers  26  does not cut through the leads. The leads have a lead height LH 0 , measured from the dashed line  3 — 3  that will be the approximate location of the media-facing surface, of about 50-100 microns. 
     After forming the back edges  50  and  52 , another read gap layer, not shown, is formed over the sensor layers  26  and lead layer  44  shown in FIG. 3. A magnetic shield layer that may optionally serve as a write pole layer, not shown, is then formed. After optional formation of a write transducer, not shown, the wafer  20  upon which perhaps a thousand of these sensors has been formed is diced into rows of sensors, one of the rows diced along the dashed line  3 — 3 . The structure shown in FIG. 4 is symmetrical about line  3 — 3 , so that a pair of sensors may be formed upon cutting along that line  3 — 3 , each of the sensors having a media-facing surface adjacent to line  3 — 3 . After further processing, including creation of a protective coating on the media-facing surface, the row is divided into individual heads for interaction with a media. 
     In an effort to increase storage density, the track width TW 0  of the sensor layers  26  may be reduced below that current commercially available range of 0.5 micron to 1.0 micron. As the track width TW 0  is reduced, however, the undercut used in the lift off process may become a larger fraction of the mask width, so that the lower mask layer  27  can no longer support the upper layer  28 . Moreover, reducing the width of mask  25  below 0.5 micron approaches the limits of conventional photolithography. 
     SUMMARY 
     In accordance with the present invention, methods are disclosed for reducing feature sizes of devices such as electromagnetic sensors. A track width of such a sensor may be defined by a mask having an upper layer with a reduced width and a lower layer with a further reduced width. Instead of or in addition to being supported by the lower layer in the area defining the sensor, the upper layer is supported by the lower layer in areas that do not define the sensor width. In some embodiments the upper layer forms a bridge mask, supported at its ends by the lower layer, and the lower layer is completely removed over an area that will become a sensor. Also advantageous is a mask having more than two layers, with a bottom layer completely removed over the sensor area, and a middle layer undercut relative to a top layer. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a cross-sectional view of a step in the formation of a conventional MR sensor. 
     FIG. 2 is a cross-sectional view of a step in the formation of the conventional MR sensor subsequent to that shown in FIG.  1 . 
     FIG. 3 is a cross-sectional view of a step in the formation of the conventional MR sensor subsequent to that shown in FIG.  2 . 
     FIG. 4 is a top view of the step in the formation of the conventional MR sensor shown in FIG.  3 . 
     FIG. 5 is a cross-sectional view of a partially completed MR sensor in accordance with the present invention. 
     FIG. 6 is a top view of the partially completed MR sensor shown in FIG.  5 . 
     FIG. 7 is a cross-sectional view of a step in the formation of the MR sensor subsequent to that shown in FIG.  5 . 
     FIG. 8 is a cross-sectional view of a step in the formation of the MR sensor subsequent to that shown in FIG.  7 . 
     FIG. 9 is a top view of the partially completed MR sensor shown in FIG.  8 . 
     FIG. 10 is a cross-sectional view of a completed MR sensor formed from the partially completed MR sensor shown in FIG.  9 . 
     FIG. 11 is a cross-sectional view of a step in the formation of a MR sensor in accordance with the present invention, including a mask that has been completely undercut in the illustrated cross-section. 
     FIG. 12 is a top view of the partially completed MR sensor shown in FIG.  11 . 
     FIG. 13 is a cross-sectional view of a step in the formation of the MR sensor subsequent to that shown in FIG.  11 . 
     FIG. 14 is a cross-sectional view of a completed MR sensor formed from the partially completed MR sensor shown in FIG.  13 . 
     FIG. 15 is a cross-sectional view of a step in the formation of a MR sensor in accordance with the present invention, including a mask that has been completely undercut and notched in the illustrated cross-section. 
     FIG. 16 is a cross-sectional view of a step in the formation of the MR sensor subsequent to that shown in FIG.  15 . 
     FIG. 17 is a cross-sectional view of a completed MR sensor formed from the partially completed MR sensor shown in FIG. 16, as seen from a cross-section adjacent a media-facing surface. 
     FIG. 18 is another cross-sectional view of the completed MR sensor of FIG. 16, as seen from a cross-section orthogonal to that shown in FIG.  17 . 
     FIG. 19 is a perspective view of a head including the MR sensor of FIG.  17  and FIG. 18, that has been opened to reveal the lead layers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 is a cross-sectional view of a partially completed MR sensor in accordance with the present invention. Over a wafer substrate  100  a magnetic shield layer  102  has been formed, either directly on the substrate or on an intermediate layer, not shown. Atop the shield layer  102  a first read gap layer  104  of dielectric materials has been formed, and atop the read gap layer a plurality of magnetoresistive (MR) sensor layers  106  has been formed. 
     The sensor layers  106  may form a spin valve sensor that includes a pinning layer that stabilizes a magnetic moment of a pinned layer, the pinned layer being separated from a free layer by a nonmagnetic spacer layer, the free layer having a magnetic moment that can vary in response to an applied field. The pinning layer may be formed of an antiferromagnetic material, synthetic antiferromagnet, or current carrying conductor. The pinned and free layers may be formed of ferromagnetic materials such as nickel-iron (NiFe), or half metallic magnet materials such chromium-oxide (CrO 2 ) or iron-oxide (Fe 3 O 4 ), and possible antiferromagnetic materials include PtMn, NiMn, PtNiMn and PtCrMn. The spacer layer may be an electrically conductive material such as copper (Cu) or gold (Au). Alternatively, the sensor layers  106  can represent any known MR sensing mechanism that can be formed in accordance with the present invention, including anisotropic, giant and colossal magnetoresistive mechanisms. More generally, sensor layers  106  represent active layers through which electromagnetic transport of electrons or photons is used to sense, store or provide information in an electromagnetic device. 
     A mask  105  has been formed of a lower layer  107  and an upper layer  108  and, after photolithographic patterning, layer  107  has been chemically removed relative to layer  108 , forming undercut edges  101  and  103 . The lower layer  107  has a width between edges  101  and  103  that is greatly reduced compared to the prior art as well as being much smaller than that of upper layer  108 . Whereas prior art undercut masks would be expected to collapse if a width of a lower layer was less than a sum of the undercut distances, the lower layer  107  can have a width that is less than half that of the upper layer  108 . A directional removal step such as ion beam etching (IBE) has been performed to create edges  111  and  113  of the sensor layers  106 , the IBE also removing part of the read gap layer  104 . 
     FIG. 6 is a top view of the partially completed MR sensor shown in FIG.  5 . As shown in FIG. 6, the lower mask layer  107  can be significantly smaller in sensor area  117  because it is significantly wider in adjacent support areas  120  and  122 . Thicker areas  120  and  122  of the lower mask  107  may be separated from each other by about 10 microns or less, allowing a thinner area  117  of that mask to have a width of less than one-quarter micron. The hourglass shaped patterns of mask layers  107  and  108  are in contrast to conventional masks that typically extend in straight lines at least 50 microns from lines that will be cut and formed into media-facing surfaces. 
     Mask layer  107  in this embodiment contains polydimethylglutarimide (PMGI) underlayer material, although other photo insensitive organic materials that are soluble in developers may alternatively be employed. Patterned photoresist layer  108  may be formed from any of several photoresist materials as are conventional in the art of MR sensor element fabrication. Such photoresist materials may be selected from photoresist materials including positive photoresist materials and negative photoresist materials. 
     FIG. 7 is a cross-sectional view of a step in the formation of the MR sensor subsequent to that shown in FIG.  5 . An electrically conductive layer  110  of hard magnetic bias material is deposited on the sensor layers  106 , read gap  104  and upper mask  108 , to provide longitudinal magnetic bias to the sensor layers  106 . An electrically conductive lead layer  112  is then deposited on the bias layer  110 , so that the bias layer and lead layer together form electrical leads for flowing current through the sensor layers  106 . 
     FIG. 8 is a cross-sectional view of a step in the formation of the conventional MR sensor subsequent to that shown in FIG.  7 . In FIG. 8, the mask layers  107  and  108  have been removed by dissolving at least the lower mask layer  107  with a known solvent. The bias layers  110  and lead layers  112  together form metallic electrical leads for the sensor layers  106 . The electrical leads are separated, in this embodiment, by a space left by mask  107 , that distance between the electrical leads forming the active width or track width TW 1  of the sensor. 
     FIG. 9 is a top view of the partially completed MR sensor shown in FIG.  8 . Sensor layers  106  have been terminated at back edges  130  and  133  by masking and IBE, not shown, exposing read gap  104  but not cutting through lead layers  112 . Cross-sectional line  8 — 8  shows the location at which the wafer substrate and thin film layers will be diced after completion of wafer level processing. 
     FIG. 10 is a cross-sectional view of the layers shown in FIG. 9, after subsequent processing including formation of additional layers and dicing of the wafer substrate and thin film layers. The layers have been polished along the diced surface and a hard coating  126 , for example made of diamond-like carbon (DLC), has been applied to create a media-facing surface  128 . Another read gap layer  134  of dielectric material such as alumina has been deposited atop the read gap  104 , not shown in this cross-sectional view. Read gap layer  134  has an area between the leads that is shaped like a profile of a wineglass adjacent the sensor layers  106 . Note that the lead layers could extend instead in other shapes provided that within about ten microns of the media-facing surface  128  they are further separated from each other than they are adjacent the media-facing surface  128 . For example, instead of the leads each having an edge distal to the media-facing surface that has a serpentine shape, the edge may have a single bend. The lead layers  112  in this cross-section have a lead height LH 1  from the media-facing surface  128  that is less than ten microns for portions of the leads that are separated from an edge of the track width TW 1  by less than two microns. Lead height LH 1  may be is less than about five microns for sections of the leads that are separated from an edge of the track width TW 1  by less than one micron. 
     FIG. 11 is a cross-sectional view of a step in forming another embodiment of a MR sensor in accordance with the present invention. Substrate  100 , shield layer  102  and read gap layer  104  may be substantially as previously described. In this embodiment, however, a mask  208  is suspended above the sensor layers  206 , separated by a void  205 . The mask  208  has a mask width MW that is less than one micron and that may be much smaller with use of high-resolution photolithography. The sensor layers  206  have been trimmed by IBE or similar processes. 
     As shown in FIG. 12, lower layer supports  207  allow mask  208  to be suspended over the sensor layers like a bridge in the vicinity of line  11 — 11 . 
     FIG. 13 shows a later step in the processing of the sensor shown in FIG.  11  and FIG. 12. A hard bias layer  210  has been deposited, followed by an electrically conductive layer  212 , after which the masks  207  and  208  and any overlying bias or conductive materials have been lifted off. A track width TW 2  of the sensor layers  206  may, similar to that of the previous embodiment, be in a range between a micron and a nanometer. Reliability and manufacturing yield may be improved, since the exact amount of undercut of the lower mask layer  207  is not critical to the track width TW 2 . 
     FIG. 14 is a cross-sectional view of the layers shown in FIG. 13, after subsequent processing including formation of additional layers and dicing of the wafer substrate and thin film layers. The layers have been polished along the diced surface and a hard coating  226 , for example made of diamond-like carbon (DLC), has been applied to create a media-facing surface  228 . The sensor layers  206  have been ion milled along a back edge  230 , which in this embodiment has not cut through the bias layers  210 . 
     Another read gap layer  234  of dielectric material such as alumina has been deposited atop the read gap  104 , read gap  104  not being visible in this cross-sectional view. Read gap layer  234  has a wineglass shape adjacent the sensor layers  206 , visible between the bias layers  210 . Other shapes for the border between read gap  234  and the leads such as bias layers  210  are possible, provided that the leads are further separated than the track width TW 2  within several microns of the media-facing surface  228 , as a remnant of the support for the bridge-like mask. Depending upon factors such as the thickness of the bias layers  210 , the lead layer portion  212  of the electrical leads may instead be disposed in the cross-section shown in FIG.  14 . Metallic leads such as bias layers  210  have a lead height LH 2  from the media-facing surface that is less than ten microns and preferably less than about five microns when measured in an area beyond the track width TW 2  but within about a micron of the center of the track width TW 2 . 
     FIG. 15 is a cross-sectional view of a step in forming another embodiment of a MR sensor in accordance with the present invention. Substrate  100 , shield layer  102  and read gap layer  104  may be substantially as previously described. In this embodiment, however, a notched mask  303  including mask layers  307  and  308  is suspended above the sensor layers  306 , separated from the sensor layers  306  by an air gap  305 . 
     The sensor layers  306  have been trimmed by IBE or similar processes. Notches  301  and  302  in mask  303  obstruct materials that are removed during IBE from being redeposited on the sensor layers  306 , as such redeposition can be harmful to operation of the sensor. That is, migration of redeposition materials over the sensor layers  306  is a function of a height to width aspect ratio of the air gap  305 . If the height of air gap  305  is reduced to prevent migration, however, subsequent deposition of bias and lead layers may envelop mask  303 , preventing lift off. Notches  301  and  302  allow the subsequent lift off, even when the height of the air gap  305  is reduced. 
     Notched mask  303  may be created, for example, using three or more mask layers atop sensor layers  306 , with a lower layer, not shown, that is removed at a greater rate than a middle layer  307 . AS an example, mask layer  308  may be formed of negative or positive photoresist that has a thickness in a range between about 0.3 micron and 0.6 micron, and a width in a range between about 0.1 micron and 1.0 micron. 
     Layer  307 , which for a positive photoresist  308  may be formed for example of inorganic materials such as AlN or Cu 2 O, may have a thickness in a range between about 0.02 micron and 0.1 micron. Air gap  305  in this case may have been formed with a 0.02 micron to 0.1 micron thick layer of PMGI, which was dissolved away in developer such as KOH that also removed some of layer  307 . 
     Alternatively for a positive photoresist layer  308 , layer  307  may be formed of an inorganic material such as a silicon-oxy-nitride (SiO x N y ), silicon oxide (SiO), silicon (Si) or hard carbon (C) materials that are removed by a reactive ion etch (RIE) that also removes the PMGI layer to form the air gap  305 . Deposition of inorganic layer  307  may be performed by sputter deposition, ion beam deposition, chemical vapor deposition or other known means for forming inorganic materials. CF 4 O 2  RIE of the inorganic layer  307  may be accomplished with CF 4 O 2  for silicon based materials, or pressurized oxygen for carbon based materials, followed by a wet etch. 
     For the situation in which a negative photoresist is used for layer  308  the entire mask  303  may be formed for example of a layer of organic materials, deposited atop a PMGI layer that is removed to form the air gap  305 . The notches  301  and  302  in this case may be formed as a result of photolithographic exposure, with the PMGI layer removed by solvent. 
     Another way to provide mask layers having different removal rates is to form the lower layer and the middle layer  307  with different concentrations of PMGI. Then, after photolithographic definition of photoresist layer  308 , which may be a negative or positive photoresist, solvent is applied that completely removes the lower layer from this cross-section, and undercuts layer  307  relative to layer  308 . Although not shown in this cross-section, middle layer  307  and upper layer  308  are supported elsewhere, allowing mask  303  to be suspended over the sensor layers like a bridge. 
     FIG. 16 shows a later step in the processing of the sensor shown in FIG.  15 . An electrically conductive bias layer  310  has been deposited, followed by an electrically conductive lead layer  312 . An advantage of using the notched mask  303  is that electrical leads such as bias layer  310  and electrically conductive layer  312  may be formed with a reduced possibility that they will completely envelope the mask  303 , which would prevent lift off. 
     FIG.  17  and FIG. 18 show cross-sections of a completed MR sensor  300  in accordance with the present invention, formed with the partially completed sensor shown in FIG.  16 . After the masks  307  and  308  and any overlying bias or conductive materials shown in FIG. 16 are lifted off, masking and IBE defines a back edge  315  of the sensor layers  306 . A back gap layer  318  of electrically insulating material may optionally be formed while the sensor layers are still masked. A track width of the sensor layers  306  may, similar to that of the previous embodiment, be in a range between a micron and a nanometer. 
     Atop the sensor layers  306  and leads  312  a second read gap layer  320  is formed, followed by a second magnetically permeable shield layer  322  that also serves as a first pole layer for an inductive transducer that is used for recording data on a media, not shown. After polishing the shield/pole layer  322  a non magnetic recording gap layer  325  is formed, followed by an electrically conductive coil layer  343 , which is surrounded with electrically insulating material. A second magnetically permeable pole layer  328  for the inductive transducer is then formed, surrounded by electrically insulating material. A protective layer  330  is then formed that will define a trailing end  333  of a read/write head including sensor  300 . The wafer substrate  100  and adjoining thin film layers is then diced and polished in the vicinity of the cross-section shown in FIG. 17, and another protective coating  323  is applied to form a media-facing surface  350  shown in FIG.  18 . 
     Alternatively, sensors in accordance with the present invention can be formed with lead structures that are created prior to sensor layers, using an undercut, bridge or notched mask as described above. 
     FIG. 19 shows a perspective view of a head  400  containing the MR sensor  300  of FIG.  17  and FIG.  18 . The head  400  has been polished on the trailing end  333  to expose lead layers  312 , removing a number of the functional layers that are shown in FIG.  17  and FIG.  18 . The head has a leading end  404  separated from the trailing end  333 , and a pair of sides  406  and  408 . The media-facing surface  350  has a plurality of air-bearing pads  410 ,  412  and  414  in this embodiment, with the sensor layers  306  disposed adjacent trailing pad  414 . 
     The leads  312  are close together adjacent to the media-facing surface  350 , separated by the track width, and further apart a few microns from the media-facing surface. Stated differently, a height of the leads adjacent to a center of the sensor track width is much less than the height of the leads at least five microns away from the center of the sensor track width. 
     Although the above description has focused on illustrating the formation of an electromagnetic sensor, other devices can be formed in accordance with the present invention. For example, an undercut, bridge or notched mask as described above can be employed in a lithographic technique to enable the production of sub-half micron conductive or nonconductive patterns on semiconductor devices such as electronic circuits. Moreover, other embodiments and modifications of this invention will be apparent to persons of ordinary skill in the art in view of these teachings. Therefore, this invention is limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.