Abstract:
A giant magnetoresistive (GMR) head is formed to include a recess in an overcoat layer that reduces stress on the poles. The process includes depositing a seed layer over the overcoat layer prior to plating a metal mask layer with an opening where the recess is to be formed, wet chemical etching the seed layer through the opening in the mask layer and performing an ion milling process to remove any remaining traces of the seed layer. With the seed layer completely removed, a trench having smooth sidewalls and bottom is etched in the overcast layer by a reactive ion etch (RIE) process. The saw that is used to separate the head elements in the wafer can be passed through the clean trench without contacting the overcoat layer, thereby avoiding the chipping and cracking that might otherwise result from the use of a silicon dioxide or silicon nitride overcoat layer.

Description:
This application is a divisional of application Ser. No. 10/857,036, filed May 28, 2004, abandoned, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to giant magnetoresistive (GMR) heads for recording and reading magnetic transitions on a moving magnetic medium. In particular, this invention relates to the problems created by the thermal expansion of the layers in such GMR heads. 
     BACKGROUND 
     In the operation of a typical GMR head device, a moving magnetic storage medium, typically a disk, is placed near the pole-tips of the GMR head. During the read operation, the changing magnetic flux from magnetized regions in the moving storage disk induces a changing magnetic flux in the pole-tips and the gap between them. The magnetic flux is carried through the pole-tips and yoke-shaped core and around spiral conductor coil winding turns located between the yoke arms. The changing magnetic flux induces an electrical voltage across the conductor coil. The electrical voltage is representative of the magnetic pattern stored on the moving magnetic storage disk. During the write operation, an electrical current is caused to flow through the conductor coil. The current in the coil induces a magnetic field across the gap between the pole-tips. A fringe field extends into the nearby moving magnetic storage disk, inducing (or writing) a magnetic domain in the magnetic storage disk. Impressing current pulses of alternating polarity across the coil causes the writing of magnetic domains of alternating polarity in the storage disk. 
     The GMR head is normally attached to a substrate, the head and substrate together forming a slider. The substrate includes aerodynamic surfaces that cause the slider to “fly” over the moving disk. 
     As the recording density of the magnetic domains in the magnetic disks increases, the “flying height” of the GMR heads has become lower. The reduced flying height is necessary to enable the head to read the data bits stored on the disk effectively and without interference or crosstalk from adjacent data bits. 
     The lessening in the flying height has created a number of problems in the fabrication of the GMR heads. One of these problems relates to the thermal properties of the layers that together make up the head. In particular, the head tends to heat up by friction with the supporting layer of air as the head “flies” over the disk, and the constituent layers expand as this happens. This expansion increases the risk that the head will contact or “crash into” the disk, thereby damaging the head, the disk, or both, and that stresses will be created between the layers in the head. 
     Typically, the top layer in the head is a relatively thick “overcoat layer” that is formed of alumina (Al 2 O 3 ). One problem with alumina is that its coefficient of thermal expansion (CTE) of 6 μm/m/° C. is relatively high, which creates a temperature-induced protrusion at the air-bearing surface (ABS) when the head heats up. Other materials with lower CTEs might be desirable as substitutes for alumina in the overcoat layer, but in many cases these other materials present manufacturability problems. 
     U.S. Pat. No. 5,643,259 to Sone et al. describes the formation of a recess in the overcoat layer at the trailing edge of the slider, which, it claims, prevents the temperature-induced protrusion from extending “above a predetermined level of the surface facing the disk” (col. 2, lines 50-51). Alumina is used for the overcoat layer, however, so Sone et al. are limited to the relatively high CTE of alumina. Published European Patent Application No. 0627732 A1 teaches an overcoat layer made of silicon dioxide or silicon nitride, both of which have a CTE less than alumina, but it fails to teach a technique for overcoming the fabrication problems presented by the use of these materials, namely, that they tend to chip or crack when the GMR head elements in a wafer are separated from each another by sawing. 
     Accordingly, what is needed is a material for use in the overcoat layer that has a CTE lower than alumina and yet can readily accommodate to the fabrication process. 
     SUMMARY 
     In accordance with an embodiment of this invention, silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) is used as an overcoat layer in a giant magnetoresistive (GMR) head, and a recess is formed in the silicon dioxide or silicon nitride overcoat layer to prevent the overcoat layer from chipping during the separation (sawing) of the wafer into individual heads and to relieve stress on the other layers of the head during operation. The recess is formed by a process that includes: depositing (e.g., plating) a seed layer on the surface of the overcoat layer, depositing a mask layer over the seed layer, the mask layer having an opening where the recess is to be located thereby exposing a section of the seed layer, wet-etching the exposed section of the seed layer through the opening in the mask layer, removing any remaining portions of the exposed section of the seed layer by a reactive ion etch (ion milling), and etching the recess in the overcoat layer through the opening in the mask layer by reactive ion etching. By this process, essentially all of the exposed section of the seed layer is removed, and this yields a recess having a smooth floor and sidewall. 
     Typically, a plurality of heads are formed on a single wafer, and the process described above is used to form a rectilinear lattice of trenches that separate the individual heads. In the finished heads, the recess is located on one side of each head. Therefore, the dicing saw that is used to separate the heads cuts a path that abuts three sides of each head and is separated from the fourth side of each head by a distance that is substantially equal to the width of the recess. After the heads have been diced, the section of the wafer than remains attached to the head becomes the substrate, and the head and substrate together form the slider. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a conventional slider containing a GMR head. 
         FIG. 2  is a side view of a slider containing a GMR head with a silicon dioxide or silicon nitride overcoat layer and a recessed overcoat layer. 
         FIG. 3  is a detailed cross-sectional view of a GMR head in accordance with the invention. 
         FIGS. 4A-4H  illustrate a process for fabricating a GMR head according to the invention. 
         FIGS. 5A and 5B  are top views of a portion of a wafer containing GMR heads prior to dicing. 
         FIG. 6A  is a cross-sectional view of a portion of the wafer before the GMR heads are separated. 
         FIG. 6B  is a cross-sectional view of a single GMR head after it has been separated from other heads in the wafer. 
         FIG. 7  is a graph showing a comparison of the temperature-induced protrusion generated by alumina and silicon dioxide overcoat layers, respectively, as a function of the thickness of the overcoat layer. 
         FIG. 8  is a graph showing the size of the temperature-induced protrusion generated by a silicon dioxide overcoat layer as a function of the width of the recess. 
         FIG. 9  is a graph showing the relative change in the temperature-induced protrusion of various layers in GMR heads containing an alumina and a silicon dioxide overcoat layer, respectively, as the temperature increases from 25° C. to 75° C. 
         FIG. 10  is a graph showing the rise time of the magnetic flux in a head containing a recessed overcoat as compared with the rise time of the magnetic flux in a head containing a non-recessed overcoat. 
         FIG. 11  is a graph showing the write current required to saturate a head containing a recessed overcoat as compared with the write current required to saturate a head containing a non-recessed overcoat. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a side view of a slider  10  and a magnetic storage disk  12 . Slider  10  includes a substrate  100 , which has a leading edge  102 , and a giant magnetoresistive (GMR) head  104 . As disk  12  moves in the direction indicated by the arrow, air strikes the leading edge  102  and causes slider  10  to “float” at a flying height H above disk  12 . 
     As indicated above and as described further below, GMR head  104  includes a number of layers of insulating and magnetic materials. One of the thickest layers is the overcoat layer, indicated at  106 . As the air passes beneath assembly  10 , friction between the moving air and assembly  10  causes assembly  10  to heat up, for example, to a temperature of 50° to 75° C. This in turn causes the layers in GMR head  104  to expand and can create a temperature-induced protrusion, represented by the dashed line in  FIG. 1 . If the temperature-induced protrusion becomes too large, contact may occur between GMR head  104  and disk  12 . Such contact, usually referred to a “crash,” can damage GMR head  104 , disk  12 , or both. 
     Overcoat layer  106  is customarily made of alumina. Alumina has a relatively high coefficient of thermal expansion (CTE) of 6 μm/m/° C. It would be preferable to use a material such as silicon dioxide, which has a CTE of 2 μm/m/° C., or silicon nitride, which has a CTE of 3 μm/m/° C. A 30-50% reduction in the size of the temperature-induced protrusion can be achieved by forming the overcoat layer of silicon dioxide or silicon nitride. 
       FIG. 2  shows a similar cross-sectional view of a slider  20 , which contains a GMR head  204  in accordance with the invention. An overcoat layer  206  in GMR head  204  is made of silicon dioxide or silicon nitride, and a recess  208  has been formed in overcoat layer  206  at the trailing edge of slider  20 . 
       FIG. 3  is a detailed cross-sectional view of GMR head  204 , showing its constituent layers. A portion of substrate  200  is also shown. Note that, in  FIG. 3 , GMR head  204  has been rotated 90° as compared with  FIG. 2 , so that recess  208  is at the upper right hand corner and substrate  200  is located under GMR head  204 . 
     The structure of GMR head  204  will now be described. Starting at the bottom, in direct contact with substrate  200  is an undercoat layer  210 , which is typically made of alumina. Layer  212  is an optional layer that may contain magneto-resistive (MR) head. In some embodiments, layer  212  is omitted. Above layer  212  are two layers  214  and  216  of a magnetic material such as NiFe that together form the bottom pole P 1  of GMR head  204 . A plurality of coil windings C 1  are formed in an opening in layer  216 , separated from layer  214  by an insulating layer  218 . Layers  220  and  222  are magnetic layers of a magnetic material such as NiFe that together form a top pole P 2 . Layer  222 , normally referred to as the yoke, is curved, and a plurality of coil windings C 2  are formed in the space created by the curve in layer  222 . Poles P 1  and P 2  are separated by an insulating layer  224  which forms a gap  226  at the air-bearing surface ABS. To write data, a current is applied through terminals (not shown) that connect to coil windings C 1  and C 2 . This current induces a magnetic field across the gap  226 , which writes data onto a magnetic data storage disk. 
     An alumina layer  228  covers magnetic layer  222 , and overcoat layer  206  is formed over alumina layer  228 . Alumina layer  228  may be 1-5 μm thick, for example. Overcoat layer  206  may be about 20 μm thick as measured from the top of alumina layer  228  (T) and about 30 μm thick as measured from the gap  226 . As noted above, overcoat layer  206  is made of silicon dioxide or silicon nitride to take advantage of the lower CTE of these materials as compared with alumina. Recess  208 , having a width W r  and a depth D r , is shown at the upper right hand corner of the figure. W r  may be equal to 3 μm±2 μm, for example. 
     Methods of fabricating poles P 1  and P 2 , coil windings C 1  and C 2  and the intervening insulating layers are well known in the art and will not be described here. 
     It is important that recess  208  be perfectly vertical and have smooth side walls and bottom, free of any spikes or projections of silicon dioxide or silicon nitride. Otherwise, during the dicing process (described below), these brittle spikes or projections will tend to break off and fragment, causing crack/chip defects and reducing product yield. Furthermore, the brittle spikes may break of inside the disk drive, causing it to fail. 
     As noted above, typically a plurality of GMR heads are fabricated simultaneously on a wafer, and then the wafer is diced (sawed) to separate the heads from each other.  FIGS. 4A-4H  illustrate a cross-sectional view of two GMR heads  260  and  262  in a wafer  250 . For the sake of clarity, the details of GMR heads  260  and  262  have been omitted. Only the contour of the yoke is shown.  FIGS. 4A-4H  will be used to explain the process of forming recesses  208 . 
     As shown in  FIG. 4A , the process starts with overcoat layer  208  having been deposited over the top surface of the wafer  250 . The thickness of overcoat layer  208  may vary from 15 μm to 45 μm, for example. Overcoat layer  208  is made of silicon dioxide or silicon nitride and may be deposited by a conventional physical vapor deposition (PVD) or plasma-enhanced chemical vapor deposition (PECVD) process. After overcoat layer  208  has been deposited, its top surface may be lapped or chemical-mechanical polished to planarize it, remove any irregularities and expose the copper connections. 
     Next, as shown in  FIG. 4B , a seed layer  270  is deposited on the surface of overcoat layer  208  to form a base for the metal mask layer that will be deposited later (see below). Seed layer  270  is typically deposited by evaporation or PVD and may be 800 Å thick in one embodiment. The composition of seed layer  270  depends on the composition of the metal mask layer that will later be deposited. Table 1 shows the composition of seed layer  270  for several types of metal mask layer. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Metal Mask Layer 
                 Seed Layer 
               
               
                   
                   
               
             
             
               
                   
                 NiFe 
                 Ta/Cu/NiFe, CoFe, CoFeN, NiCr 
               
               
                   
                 CoFe 
                 NiCr 
               
               
                   
                 CoNiFe 
                 NiCr 
               
               
                   
                 Cu 
                 AuCr, Cu, NiFe/Cu 
               
               
                   
                   
               
             
          
         
       
     
     It should be understood that the seed layer materials listed in Table 1 are illustrative only and not limiting. Other conductive metals can be used as the seed layer. 
     After seed layer  270  has been deposited, a photoresist layer  272  is formed on seed layer  270  and patterned as shown in  FIG. 4C . The sections of photoresist layer  272  that remain after patterning cover the areas where recesses  208  are to be formed. As shown in  FIG. 3 , recesses  208  include the plane defined by the ABS and extend back a distance W r  from the ABS. 
     As shown in  FIG. 4D , a metal mask layer  274  is deposited in the areas not covered by photoresist layer  272 . Metal mask layer  274  may be deposited by plating or physical vapor deposition (sputtering) and may be 1-2 μm thick, for example. Metal mask layer  274  may consist of any of the materials listed in Table 1. After metal mask layer  274  has been deposited, the remaining portions of photoresist layer  272  are removed, yielding the structure shown in  FIG. 4E , with openings  276  formed in metal mask layer  274  where recesses  208  will be located. 
     As  FIG. 4E  indicates, the formation of openings  276  in metal mask layer  274  exposes areas of seed layer  270  which must be removed before overcoat layer  206  can be etched. It is highly important that all of the areas of seed layer  270  that lie beneath openings  276  be removed while doing minimal damage to the vertical surfaces of metal mask layer  274  that surround openings  276 . Known processes of removing seed layer  270 , which use a wet-chemical etch, leave traces of seed layer  270  in the openings  276 . These traces act as “micromasks” during the etching of overcoat layer  206 , creating a jagged surface which is replicated as the etching of overcoat layer  206  continues down to the etch-stop layer (e.g. alumina layer  228  in  FIG. 3 ). The resulting spikes and other irregularities in overcoat layer  206  can break off or fracture during the dicing of wafer  250 . 
     To insure that all of the seed layer  270  that is exposed by openings  276  is removed, a combination wet/dry etch process is used. First, seed layer  270  is exposed to a wet chemical etch. The chemicals and temperature of the etch process depend on the composition of seed layer  270  and are shown in Table 2. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Seed Layer 
                 Etchants 
                 Temperature 
               
               
                   
               
             
             
               
                 CoFe or CoFeN 
                 30-50% HCl and Ferrous Sulfate 
                 20-35° C. 
               
               
                 NiFe 
                 H 2 SO 4  and Ferric Ammonium Sulfate 
                 20-35° C. 
               
               
                 Cu 
                 25-45% NaH 4 OH and Ammonium 
                 15-35° C. 
               
               
                   
                 Persulfate 
               
               
                   
               
             
          
         
       
     
     The HCl/ferrous sulfate etchant will etch a CoFe/CoFeN seed layer without etching plated NiFe, even after etching over 6 minutes. A typical etch for a 1500 Å seed layer may last 1 to 3 minutes. 
     The H 2 SO 4 /ferric ammonium sulfate etchant will attack the NiFe seed as well as plated NiFe. It will attack Cu at a different rate. Therefore, Cu should be used as the metal mask layer. Typically, 30 seconds to 1 minute is required to etch a 1500 Å seed layer. 
     The NaH 4 OH/ammonium persulfate etchant will also attack NiFe, but at a different rate than Cu. Typically, 15 to 45 seconds are required to etch a 1500 Å Cu seed layer. 
     The wet-chemical etch is followed by a low-rate ion milling process, which cleans the interface between seed layer  270  and overcoat layer  206 . The angle of the ion beam with respect to the normal of the surface of overcoat layer  206  is set from −10° to −70°. The ion milling process can be performed for 5 to 10 minutes. 
     With all traces of seed layer  270  removed from the surface of overcoat layer  206 , the etching of overcoat layer  206  though openings  276  in metal mask layer  274  can begin. Overcoat layer  206  is etched using a reactive ion etch (RIE) process. A Unaxis etcher may be used. The RIE may be CF 4  based, with the RF power at 100-300 W. Under these process conditions, the silicon dioxide or silicon nitride etches at a rate of 4000 Å/min. If overcoat layer  206  is formed of silicon nitride, 20-100 sccm of CHF 3  are added to the RIE. 
     A CF 4 -based RIE process provides a vertical 90-degree profile. The unique wet/dry seed layer removal ensures a smooth sidewall. The wet chemical etch removes the seed layer with minimal damage to the mask layer, while the subsequent low rate ion milling process removes any remaining trace of the seed layer. The resulting sidewall of the SiO 2  or Si 3 N 4  overcoat layer is much smoother than can be obtained using a conventional seed layer removal process. 
     At the completion of the RIE process, trenches  278  have been formed in overcoat layer  206 , as shown in  FIG. 4G . Trenches  278  could be 30-120 μm wide and 5-45 μm deep, for example. Metal mask layer  274  and seed layer  270  are then removed, leaving the structure shown in  FIG. 4H . 
       FIG. 5A  is a top view of wafer  250 , showing the rectilinear lattice of trenches  278  surrounding GMR heads  260  and  262 . The upper coil C 2  and ABS of each head are shown, as well as terminals  290 , which are connected to coils C 1  and C 2  via connectors (not shown). The dashed lines in  FIG. 5B  show the paths  292 ,  294 ,  296  of the dicing saw blade that is used to separate the heads in wafer  250  on the sides adjacent the air bearing surfaces (ABS). Saw paths  298 ,  300  separate the heads on the sides perpendicular to the ABS. After the heads are separated by sawing, the edges are lapped as necessary to provide the desired width of recesses  208  (the dimension W r  in  FIG. 3 ). 
       FIG. 6  is a cross-sectional view of saw paths  292  and  298 , showing how they run along the sides of head  260 . 
     Thus, because of the formation of trenches  278 , the dicing saw does not need to cut into overcoat layer  206  which, being made of silicon dioxide and silicon nitride, is prone to chipping and cracking. Moreover, as described above, certain of the trenches  278  are used to form recesses  208 .  FIG. 6B  shows head  260  after it has been separated from the surrounding heads, with trench  278  forming recess  208 . 
     As indicated above, the use of a silicon dioxide or silicon nitride overcoat layer reduces the temperature-induced protrusion of the overcoat layer, as compared with an overcoat layer made of alumina.  FIG. 7  is a graph showing the temperature-induced protrusion of silicon dioxide and alumina layers as a function of the thickness of the overcoat layer. It indicates that the advantages of using silicon dioxide increase as the thickness of the overcoat layer increases. For example, at a thickness of 20 μm the temperature-induced protrusion of a silicon dioxide overcoat layer is less than 1.0 nm for silicon dioxide versus almost 2.0 nm for alumina. 
       FIG. 8  is a graph showing the temperature-induced protrusion of a silicon dioxide overcoat layer as a function of the width of the recess (measured from the ABS). As indicated, the temperature-induced protrusion increases as the width of the recess increases. It has been found that a recess having a width of 3±2 μm is desirable to maintain a relatively low and stable pole-top recess (PTR). 
       FIG. 9  is a graph showing the relative change in the temperature-induced protrusion of four of the layers in a GMR head as the temperature increases from 25° C. to 75° C. The four layers are the undercoat layer (UC), the pole layers P 1  and P 2 , and the silicon dioxide overcoat layer (OC). Note that the PTR of the silicon dioxide overcoat layer has been reduced about 3 nm as the temperature increases from 25° C. to 75° C. 
     This invention allows GMR head designers to obtain the reduced temperature-induced protrusion of a silicon dioxide or silicon nitride layer without sacrificing the advantages of having a recess in the overcoat layer. The formation of a recess reduces the stress to which the upper pole-tip P 2 /P 3  is exposed. The overcoat layer is generally deposited under a compressive stress. Therefore, with no recess the pole-tip P 2 /P 3  experiences a tensile stress. Computer modeling studies indicate that the pole-tip sees a stress that is equal to approximately one-half of the stress in the overcoat layer. 
     When a recess is formed in the overcoat layer, the pole-tips experience a compressive stress that is equal to about one-half of the stress in the overcoat layer. Thus the net change in the stress experienced by the pole-tips from the addition of the recess is approximately equal to the stress in the overcoat layer. 
     As shown in  FIGS. 10 and 11 , this change significantly improves the magnetic performance of the head.  FIG. 10  shows that the magnetic flux rise time (t r ) of a head having a recessed overcoat is significantly less than the rise time of a non-recessed overcoat head.  FIG. 11  indicates that the write current required to saturate a recessed overcoat head is significantly less than the write current required to saturate a non-recessed overcoat head 
     In summary, this invention permits the use of silicon dioxide or silicon nitride as an overcoat layer in a GMR head, with the consequent reduction in CTE and temperature-induced protrusion, without creating any fabrication problems and without sacrificing the electromagnetic performance of the head.