Abstract:
For PMR (Perpendicular Magnetic Recording) design, one of the major technology problems is the use of CMP to fabricate the pole structure. If the device is under-polished there is a danger of leaving behind a magnetic shorting layer while if it is over-polished there may be damage to the main pole. This problem has been overcome by surrounding the main pole, write gap, stitched write head pillar with a layer of CMP etch stop material which, using optical inspection alone, allows CMP (performed under a first set of conditions) to be terminated just as the stitched write head gets exposed. This is followed by a second CMP step (performed under a second set of conditions) for further fine trimming of the stitched head, as needed.

Description:
FIELD OF THE INVENTION 
   The invention relates to the general field of magnetic write head manufacture with particular reference to main pole trimming. 
   BACKGROUND OF THE INVENTION 
   During PMR (Perpendicular Magnetic Recording) manufacture it becomes necessary to polish all the magnetic materials away after PPT (partial pole trim) on top of the yoke area. The CMP (chemical mechanical polishing) process must be well controlled in such cases since a magnetic shorting path will be present if under-polish occurs while, on the other hand, the main pole will be damaged if excessive over-polish takes place. 
   Typically, before polish, one is faced with a stack of metal films covered by a full film of alumina. In this context, the goal of properly executed CMP (Chemical Mechanical Polishing) is to stop at a particular layer that is in the middle of whole stack of other layers so it becomes a critical and challenging task to monitor and control the CMP process in order to make a PMR head successfully. 
   Referring now to  FIG. 1  we show there an example of a magnetic write head that includes a main pole  16 , a non-magnetic write gap  15 , and a stitched write shield  14 , which together surround the coil area  17 . Located atop  14  are first and second write shields  12  and  13  respectively. All subsequent figures will be ABS (air bearing surface) views i.e. in the direction shown by arrow  18 . 
   For PMR design, one of the major technology problems is the use of CMP to fabricate the pole structure. Since plated magnetic materials are part of the structure, they must be removed completely at he conclusion of CMP. The prior art CMP process is shown as below, but, as will be seen, one encounters several unique difficulties when doing so. 
   As seen in  FIG. 2 , the PMR structure before polishing includes metal stack  14 / 15 / 16 , which was formed inside photoresist mold  22 , as well as field area  21  (made pf magnetic material). The CMP final target is to be stopped at a desired metal interface. If under-polish occurs, a magnetic shorting path will be present. On the other hand, the main pole will be damaged if over-polish occurs. Thus to maintain high yields of functional devices, CMP must be well controlled both in terms of uniformity as well as precise termination when some desired metal layer is reached. 
   As seen in  FIG. 3 , once the device and field areas have been formed, photoresist mold  22  is removed and replaced with photoresist layer  32  which extends outwards from the device so as to partially cover field area  21  as well. In this way, device pedestal  14 / 15 / 16  is all that remains. 
   It follows from the above that a monitoring scheme must be provided in order to control the CMP process tightly. However, requiring the CMP process to stop at a middle layer is completely contrary to traditional magnetic head CMP practice. FIB (focused ion beam) is commonly utilized in wafer processing to cut devices and to check where CMP stopped. This has several disadvantages: (1) very expensive due to long off-line cycle time and possible permanent damage the device (2) limited sample size (3) a high quality FIB image is obtained only if the stopping layer shows high contrast relative to other materials under removal by the ion beam which, for most applications, is not the case. 
   In other words, monitoring and controlling a CMP process in order to make PMR head successfully is a critical and a challenging task. 
   A routine search of the prior art was performed with the following references of interest being found: 
   In U.S. Pat. No. 6,063,306, Kaufman et al. disclose two slurries having different selectivities for copper and tantalum. In U.S. Pat. No. 6,554,878, Dill, Jr. et al. show different slurries to polish different materials, including alumina. Boggs et al. describe indicator areas on a wafer to check CMP results in U.S. Pat. No. 5,972,787 and in U.S. Pat. No. 6,226,149, Dill, Jr. et al. disclose the use of CMP in forming a pole while in U.S. Pat. No. 6,024,886, Han et al. teach a CMP process using a polish stop layer. 
   SUMMARY OF THE INVENTION 
   It has been an object of at least one embodiment of the present invention to provide an improved process for trimming the stitched write shield of a magnetic read head. 
   Another object of at least one embodiment of the present invention has been to be able to skip the field etch step used in the prior art. 
   Still another object of at least one embodiment of the present invention has been to be able to precisely trim the stitched write shield without damaging the main pole. 
   A further object of at least one embodiment of the present invention has been to provide a method, suitable for in-line use during full wafer production, to terminate CMP at precisely a desired level. 
   These objects have been achieved by using a plated hard mask to etch out the main pole, write gap, and stitched write shield. The resulting pillar is surrounded by a layer of CMP etch stop material, such as Ru or Ta, which, using optical inspection alone, allows CMP (performed under a first set of conditions) to be terminated just as the stitched write head gets exposed. This is followed by a second CMP step (performed under a second set of conditions) for further fine trimming of the stitched head, as needed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a typical write head showing, in particular, the main pole and stitched write shield (separated by the write gap). 
       FIGS. 2–4  (these, and all subsequent figures, being ABS views) show how pole trimming was achieved in the prior art. 
       FIG. 5  shows the starting point for the process of the present invention. 
       FIG. 6  shows how the main pole, write gap, and stitched write shield layers are formed. 
       FIG. 7  illustrates the hard mask that is used to define the read head stack. 
       FIG. 8  shows the structure after etching down as far as the substrate level. 
       FIG. 9  illustrates how CMP may be terminated precisely at the point when the stitched write head is exposed. 
       FIG. 10  illustrates the completed structure, including the two upper write shields. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In order to control CMP uniformity for this application, several process sequences have been modified: 
   (1) The field etch process ( FIG. 4 ) is skipped in order to have better topology across the whole wafer before polish. Field etch is commonly implemented right after electro-deposition of pole materials, and then the pole trimming process begins. As a result, the topology across the whole wafer varies considerably between field area and device area due to different etch rates for different materials. Additionally the dense metal features are separated quite far apart by areas of sputtered Al 2 O 3  which does not favor CMP uniformity control. 
   (2) For the present invention, we Insert a barrier layer in the middle of metal film stack and then two different, highly selective slurries are employed in two separate steps during CMP. In order to make the process stop at a desired metal layer in a film stack, we incorporate either Ru or Ta into the film stack. CMP begins with the first slurry which cuts Al 2 O 3  much faster than it does the magnetic film. After this step, the remaining metal thickness mainly depends on the plating and IBE process. Once all the magnetic materials are exposed, the second slurry, which has high selectivity for magnetic materials relative to Ru or Ta is introduced to remove bulk metal full film until the Ru (or Ta) is reached. Thus, all magnetic materials on top of the yoke will be removed with no significant damage of pole structure. 
   (3) To effect step (2), monitoring sites are provided around every device to check if the barrier layer has been reached. This offers an efficient on-line scheme to control the CMP process since the point at which the desired layer is reached during CMP may be checked by using only an optical microscope. As a result, the process can be efficiently run in a production environment. 
   (4) CMP Process 
   (i) Above all, the wafer process has been much simplified by skipping the field etch. Most of the field areas and device areas are at a similar level when processed according to the present invention. Therefore “peaks” and “valley” are uniformly distributed across the whole incoming wafer. Additionally, by using the slurry MH-881 in the first polishing step, the plated CoNiFe in the field area acts as a natural stop because the slurry removes Al 2 O 3  much faster. This facilitates subsequent control of the uniformity of the remaining magnetic film because the majority of CMP variation with metal thickness is eliminated during this step; 
   (ii) Use of two slurries with high selectivities for CoNiFe/Al 2 O 3  and CoNiFe/Ru(Ta) also improves CMP process control with respect to uniformly stopping polishing at a barrier layer. The second slurry, MH-814, removes CoNiFe and Al 2 O 3  much faster than Ru/Ta after all magnetic films are exposed. Therefore an over-polishing ‘window’ is opened which allows one to clean all CoNiFe from on top of the yoke across the whole wafer without breaking into any desired layers. 
   (iii) This invention teaches a monitoring test sites scheme around devices and overcomes all the disadvantages, discussed above) if FIB is employed to monitor the CMP process. First, the exposure check can be done on-line in real time to save wafer processing time; secondly there are four monitoring sites around every devices for sample size consideration; thirdly it is very easy to see if the desired barrier layer has been reached, even if the barrier and removal layers have limited optical contrast. This is because the Al 2 O 3  is transparent so one can look through it down to substrate. An additional advantage of this process is that as a larger fraction of the barrier layer material becomes exposed they acting to further improve CMP process uniformly. 
   Referring now to  FIG. 5 , we begin an overview description of the process of the present invention. The process begins with the provision of substrate  11  (which in practice would be Al 2 O 3 ) on which magnetic disk  16  is formed and then abutted with layer of insulation  51  (for which suitable materials include Al 2 O 3  and SiO 2 . Magnetic disk  16  is a material such as CoFeN, CoFe, or CoNiFe and it is between about 0.2 and 0.4 microns thick. This is followed by the deposition of non-magnetic write gap layer  15  (made of materials such as Al 2 O 3 , SiO 2  or Ru and deposited to a thickness between about 500 and 1,000 Angstroms) onto the coplanar surfaces of  51  and  16 . Then, magnetic layer  14  (which will later become the stitched write shield) is deposited onto layer  15 . Layer  14  is a material such as CoFeN or CoFe and it is between about 3,000 and 5,000 Angstroms thick. 
   Referring next to  FIG. 6 , magnetic disk  14  is formed out of layer  14  and then abutted by end point detection layer  61 , said layers having coplanar top surfaces on which hard mask  71  is formed ( FIG. 7 ). The process for forming the hard mask involves first laying down a photoresist mold (similar to mold  22  in  FIG. 2 ) and then electro-depositing the hard mask material onto all unprotected surfaces, following which the photoresist mold is removed. The resulting hard mask has two main parts—a central portion  71   a  that defines the shape and position of the stitched write shield  14  and the main pole  16  (see  FIG. 1 ) and a surrounding portion  71   b  to limit the size of layer  61 . 
   Moving now to  FIG. 8 , all material not covered by the hard mask is then removed down as far as substrate  11 , thus forming cavity  81  which is overfilled with insulating layer  91  (a material such as Al 2 O 3  or SiO 2 ). This is followed by the first of two CMP steps discussed earlier in which CMP proceeds until the end point detection layer which acts as an etch stop layer  61  is just exposed, the exact point at which to terminate CMP being determined through optical inspection of layer  61 . The precise exposure of layer  61  also leads to the simultaneous exposure of layer  14  so there is no danger of over-polishing the stitched write shield, as illustrated in  FIG. 9 . The first CMP step further comprises: using a slurry of 2–5 weight % alumina, with 93–96 weight % deionized water, at a pH of about 7.5 to 8.5; applying a back pressure −6 and 6 p.s.i.; having a wafer rotation speed about 50 and 70 r.p.m.; exerting a wafer polish pressure between about 4 and 6 p.s.i.; and including less than about 2% additives by weight. 
   The structure is now ready for the final trimming of stitched write shield  14  which is accomplished using the second of the two afore-mentioned CMP steps until the desired final thickness is reached. As shown in  FIG. 10 , the process concludes with the deposition of first and second write shields,  12  and  13  respectively, on layer  91  and  14 . The second CMP step further comprises: using a slurry of 2–7.8 weight % alumina, with 90–95 weight % deionized water, at a pH of about 4 to 4.8; applying a back pressure between −6 and 6 p.s.i.; having a wafer rotation speed between about 50 and 70 r.p.m.; exerting a wafer polish pressure between about 4 and 6 p.s.i.; and including less than about 3% additives by weight. 
   In summary, the present invention provides: 
   
       
       (1): a simplified process sequence by skipping field etching and its associated photo processes; 
       (2): a well controlled CMP process is achieved by using either Ta or Ru as a barrier layer; 
       (3): an accurate monitoring method is provided for polishing a full wafer.