Abstract:
The present invention provides a method of manufacturing a magnetoresistive read head which reduces electrostatic discharge and allows measurement of gap resistances in the head.

Description:
This is a division of application Ser. No. 09/753,804 filed Jan. 2, 2001, now U.S. Pat. No. 6,678,127. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for reducing electrostatic discharge (ESD) damage in thin film read heads which enables measurement of gap resistances and, more particularly, to such a device and method wherein the resistance of first and second gap layers can be measured in parallel or the resistance of each of the first and second gap layers can be measured separately. 
     2. Description of the Related Art 
     The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions. 
     An exemplary high performance GMR read head employs a spin valve sensor for sensing the magnetic field signals from the rotating magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The sensor and the first and second leads are located between first and second dielectric read gap layers which are, in turn, located between ferromagnetic first and second shield layers. Accordingly, the GMR head is electrically isolated from the two shields by the first and second gap layers which are typically aluminum oxide (Al 2 O 3 ). The gap length, which is the distance between the shield layers, is continually being shortened in order to achieve higher areal density. For a given sensor thickness, therefore, the gap layers have to become thinner. In head designs, the shields are typically not electrically connected to any other conductors on the slider, and are electrically isolated from each other. As a result, a charge may accumulate on the shields during processing. The presence of this charge causes a potential difference across the gap layers. When this voltage reaches a sufficiently high value, the dielectric breaks down, and electrical shorts can occur at the location of the breakdown. This is a type of electrostatic discharge (ESD) damage. Shorts between the sensor and the shields are detrimental to the operation of the head. A typical specification on the resistance between the shields and the sensor is 100 kOhms. Accordingly, any head with a resistance less than 100 kOhms between the read sensor and either shield fails such a test. Losses at wafer final test due to shield shorts can be as high as 30%. One way to prevent the charging of the shields is to electrically short both shields to one side of the sensor via a lead and then remove the short during slider fabrication. While this will provide protection against process-induced charging, it does not allow the ability to test for shield shorts due to other phenomena, such as pinholes in the gap dielectric. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device and method of reducing ESD damage to the sensor of the read head while enabling measurement of the first and second gap resistances. The first read gap layer can be considered to have a resistance R G1  between the first shield layer and one of the first and second lead layers and the second read gap layer can be considered to have a resistance R G2  between the second shield layer and one of the first and second lead layers. A short is provided via a plurality of resistors between a first node and each of the first and second shield layers wherein the plurality of resistors includes at least first and second resistors R S1  and R S2  and the first node is connected to either one of the first and second leads. A second node is located between the first and second resistors R S1  and R S2 . An operational amplifier has first and second inputs connected to the first and second nodes respectively so as to be across the first resistor R S1  and has an output connected to the first node for maintaining the first and second nodes at a common voltage potential. 
     In one embodiment of the invention the first and second shield layers are shorted together. In this embodiment a test instrument can be employed for determining the combined parallel resistance of the first and second gap layers by having a first side of the test instrument connected to the first node and a second side connected to each of the first and second shield layers. In another embodiment of the invention the second resistor R S2  is connected between the second node and the shield layer and a third resistor R S3  is connected between the second node and the first shield layer. In this embodiment the test instrument has a first side connected to the first node and a second side connected to the first shield layer for determining the resistance of the first gap layer separately. Alternatively, the test instrument can be employed with its first side connected to the first node and its second side connected to the second shield layer so that the resistance of the second gap layer can be determined separately. In another aspect of the invention the sensor and the resistors R S1  and R S2  or R S1 , R S2  and R S3  are coplanar. This is accomplished by forming a layer of sensor material on a wafer and then patterning the layer of material to individually form the sensor and each of the resistors. The formation of the sensor material layer can be by sputter deposition and the patterning may be accomplished by photolithography. 
     An object of the present invention is to reduce ESD damage to the sensor of a read head while enabling measurement of gap resistances in parallel of separately. 
     Another object is to accomplish the foregoing object with the sensor and a plurality of resistors patterned from a common material layer wherein the plurality of resistors are in parallel with the resistances of the first and second gap layers. 
     Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an exemplary magnetic disk drive; 
         FIG. 2  is an end view of a slider with a magnetic head of the disk drive as seen in plane  2 — 2  of  FIG. 1 ; 
         FIG. 3  is an elevation view of the magnetic disk drive wherein multiple disks and magnetic heads are employed; 
         FIG. 4  is an isometric illustration of an exemplary suspension system for supporting the slider and magnetic head; 
         FIG. 5  is an ABS view of the magnetic head taken along plane  5 — 5  of  FIG. 2 ; 
         FIG. 6  is a partial view of the slider and a piggyback magnetic head as seen in plane  6 — 6  of  FIG. 2 ; 
         FIG. 7  is a partial view of the slider and a merged magnetic head as seen in plane  7 — 7  of  FIG. 2 ; 
         FIG. 8  is a partial ABS view of the slider taken along plane  8 — 8  of  FIG. 6  to show the read and write elements of the piggyback magnetic head; 
         FIG. 9  is a partial ABS view of the slider taken along plane  9 — 9  of  FIG. 7  to show the read and write elements of the merged magnetic head; 
         FIG. 10  is a view taken along plane  10 — 10  of  FIGS. 6  or  7  with all material above the coil layer and leads removed; 
         FIG. 11  is an enlarged isometric illustration of a read head which has a spin valve sensor; 
         FIG. 12  is a circuit diagram of one embodiment of the present invention; 
         FIG. 13  is the same as  FIG. 12  except a test instrument is employed to measure the combined parallel resistance of the first and second gap layers; 
         FIG. 14  is a circuit diagram of a second embodiment of the present invention with the test instrument measuring the combined parallel resistance of the first and second gap layers; 
         FIG. 15  is a circuit diagram of a third embodiment of the present invention with the test instrument measuring the resistance of only the first gap layer; 
         FIG. 16  is the same as  FIG. 15  except that the test instrument is measuring the resistance of only the second gap layer; 
         FIG. 17  is an isometric illustration of rows and columns of magnetic heads on a wafer substrate; and 
         FIG. 18  is an exemplary plan layout of the embodiments shown in  FIGS. 12 ,  13  and  14 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Magnetic Disk Drive 
     Referring now to the drawings wherein like reference numerals designate like or similar parts throughout the several views,  FIGS. 1-3  illustrate a magnetic disk drive  30 . The drive  30  includes a spindle  32  that supports and rotates a magnetic disk  34 . The spindle  32  is rotated by a spindle motor  36  that is controlled by a motor controller  38 . A slider  42  has a combined read and write magnetic head  40  and is supported by a suspension  44  and actuator arm  46  that is rotatably positioned by an actuator  47 . A plurality of disks, sliders and suspensions may be employed in a large capacity direct access storage device (DASD) as shown in FIG.  3 . The suspension  44  and actuator arm  46  are moved by the actuator  47  to position the slider  42  so that the magnetic head  40  is in a transducing relationship with a surface of the magnetic disk  34 . When the disk  34  is rotated by the spindle motor  36  the slider is supported on a thin (typically, 0.05 μm) cushion of air (air bearing) between the surface of the disk  34  and the air bearing surface (ABS)  48 . The magnetic head  40  may then be employed for writing information to multiple circular tracks on the surface of the disk  34 , as well as for reading information therefrom. Processing circuitry  50  exchanges signals, representing such information, with the head  40 , provides spindle motor drive signals for rotating the magnetic disk  34 , and provides control signals to the actuator for moving the slider to various tracks. In  FIG. 4  the slider  42  is shown mounted to a suspension  44 . The components described hereinabove may be mounted on a frame  54  of a housing  55 , as shown in FIG.  3 . 
       FIG. 5  is an ABS view of the slider  42  and the magnetic head  40 . The slider has a center rail  56  that supports the magnetic head  40 , and side rails  58  and  60 . The rails  56 ,  58  and  60  extend from a cross rail  62 . With respect to rotation of the magnetic disk  34 , the cross rail  62  is at a leading edge  64  of the slider and the magnetic head  40  is at a trailing edge  66  of the slider. 
       FIG. 6  is a side cross-sectional elevation view of a piggyback magnetic head  40 , which includes a write head portion  70  and a read head portion  72 , the read head portion employing a sensor  74 .  FIG. 8  is an ABS view of FIG.  6 . The sensor  74  is sandwiched between nonmagnetic electrically insulative first and second read gap layers.  76  and  78 , and the read gap layers are sandwiched between ferromagnetic first and second shield layers  80  and  82 . In response to external magnetic fields, the resistance of the sensor  74  changes. A sense current I S  conducted through the sensor causes these resistance changes to be manifested as potential changes. These potential changes are then processed as readback signals by the processing circuitry  50  shown in FIG.  3 . 
     The write head portion  70  of the magnetic head  40  includes a coil layer  84  sandwiched between first and second insulation layers  86  and  88 . A third insulation layer  90  may be employed for planarizing the head to eliminate ripples in the second insulation layer caused by the coil layer  84 . The first, second and third insulation layers are referred to in the art as an “insulation stack”. The coil layer  84  and the first, second and third insulation layers  86 ,  88  and  90  are sandwiched between first and second pole piece layers  92  and  94 . The first and second pole piece layers  92  and  94  are magnetically coupled at a back gap  96  and have first and second pole tips  98  and  100  which are separated by a write gap layer  102  at the ABS. An insulation layer  103  is located between the second shield layer  82  and the first pole piece layer  92 . Since the second shield layer  82  and the first pole piece layer  92  are separate layers this head is known as a piggyback head. As shown in  FIGS. 2 and 4 , first and second solder connections  104  and  106  connect leads from the sensor  74  to leads  112  and  114  on the suspension  44 , and third and fourth solder connections  116  and  118  connect leads  120  and  122  from the coil  84  (see  FIG. 10 ) to leads  124  and  126  on the suspension. 
       FIGS. 7 and 9  are the same as  FIGS. 6 and 8  except the second shield layer  82  and the first pole piece layer  92  are a common layer. This type of head is known as a merged magnetic head. The insulation layer  103  of the piggyback head in  FIGS. 6 and 8  is omitted. 
       FIG. 11  is an isometric ABS illustration of the read head  72  shown in  FIGS. 6  or  8 . The read head  72  includes the spin valve sensor  74 . First and second hard bias and lead layers  134  and  136  are connected to first and second side edges  138  and  140  of the sensor. This connection is known in the art as a contiguous junction and is fully described in commonly assigned U.S. Pat. No. 5,018,037 which is incorporated by reference herein. The first hard bias and lead layers  134  include a first hard bias layer  140  and a first lead layer  142  and the second hard bias and lead layers  136  include a second hard bias layer  144  and a second lead layer  146 . The hard bias layers  140  and  144  cause magnetic fields to extend longitudinally through the sensor  74  for stabilizing the magnetic domains therein. The sensor  74  and the first and second hard bias and lead layers  134  and  136  are located between nonmagnetic electrically insulative first and second read gap layers  148  and  150 . The first and second read gap layers  148  and  150  are, in turn, located between ferromagnetic first and second shield layers  152  and  154 . 
     The gap length, which is the distance between the first and second shield layers  152  and  154  in  FIG. 11 , determines the linear bit read density of the read head. The linear bit density is quantified as bits per inch (BPI) which is the number of bits that can be read by the read head along an inch of a track on a rotating magnetic disk. The width of a free layer (not shown) in the sensor  74  defines the track width of the read head. The track width density is quantified as the number of tracks per inch (TPI) along a radius of the rotating magnetic disk. The product of the linear bit density and the track width density is the areal density of the read head. The higher the areal density, the higher the storage capacity of the magnetic disk drive. 
     In order to increase the linear bit density it is necessary to decrease the thicknesses of the first and second gap layers  148  and  150 . When these gap layers are made thinner there is a risk of a pinhole in a gap layer which permits an electrostatic discharge (ESD) to occur between either of the first and second shield layers and the sensor  74  or either of the first and second lead layers  134  and  136 . An ESD can destroy the spin valve sensor  74  rendering the read head inoperable. A charge can build up on either of the first and second shield layers  152  or  154  by human handling or contacting a charged object which is typically made of plastic. The risk of an ESD is primarily during fabrication of the magnetic head and mounting it on a magnetic disk drive. After mounted on a magnetic disk drive the risk of an ESD is minimal. In order to minimize ESD damage to the read sensor  74  the first and second shield layers  150  and  154  may be shorted to either of the lead layers  134  and  136 . After assembly of the magnetic head on a magnetic disk drive the short may be deleted by severing a delete pad on the surface of the slider with a laser beam. Alternatively, the circuitry for the short may be lapped away at a row level of magnetic heads before dicing the row into individual heads and assembly on the magnetic disk drive. While a short between the first and second shield layers and either one of the first and second lead layers  134  and  136  minimizes ESD damage to the sensor  130 , there has been no provision for determining the resistances of the first and second gap layers  148  and  150  and rejecting heads which have low resistances due to pinholes in either of the gap layers. 
     First Embodiment of the Invention 
     A first embodiment  200  of the present invention is shown in  FIG. 12  which shows the first and second lead layers  134  and  136  (L 1  and L 2 ) connected to the read sensor  74 . The sensor  74  is shown as having a resistance R MR .  FIG. 12  also shows the first and second shield layers S 1  and S 2   80  and  82  are shorted by a lead  202 . First and second resistors  204  and  206  are connected across the second lead L 2  and the first and second shield layers S 1  and S 2 . With this arrangement the first read gap layer  76  has a resistance R G1  between the shield layers S 1  and S 2  and the second lead L 2  and the second read gap layer  78  has a resistance R G2  between the shield layers S 1  and S 2  and the second lead L 2 . Alternatively, the first and second resistors  204  and  206  may be connected between the first and second shield layers S 1  and S 2  and the first lead layer L 1 . In this instance, the resistance R G1  would be the resistance between the shield layers S 1  and S 2  and the first lead layer L 1  and the resistance R G2  would be the resistance between the shield layers S 1  and S 2  and the first lead layer L 1 . A center point (CP) is located between the first and second resistors  204  and  206  which will be discussed in more detail hereinafter. 
       FIG. 13  is the same as  FIG. 12  except a circuit tester  208  is connected across the first and second shield layers S 1  and S 2  and the second lead layer L 2 . The circuit tester  208  applies a predetermined voltage or a predetermined current and then reads the current or the voltage respectively. Assuming the circuit tester  208  applies a predetermined voltage and reads the current, then the resistance of the circuit, which is the parallel combination of R S1  plus R S2 , R G1  and R G2 , is the predetermined voltage divided by the current. It should be noted that no current flows through the sensor  74  since the first lead L 1  is floating. The resistance value of the series combination R S1  plus R S2  can be made high enough so that it is roughly equal to or larger than any shield short of interest. Assuming, however, that the specification on shield shorts is 100 kOhm, a resistance that high may not offer sufficient protection from shield charging effects and would require a very long resistor. This problem is overcome in the second embodiment. 
     Second Embodiment of the Invention 
       FIG. 14  illustrates a second embodiment  300  of the present invention which is an improvement over the first embodiment  200  in FIG.  13 . The embodiment  300  is the same as the embodiment  200  except for the following. The center point (CP) has two separate connections  302  and  304 . The second lead L 2  can be considered as a first node in the circuit and the connection  302  can be considered as a second node. An operational amplifier  306  has a first input  308  connected to the second lead L 2  (first node) and a second input  310  connected to the first contact  302  (second node). The output  312  of the operational amplifier is connected to the second contact  304  which is located between the first contact  302  and the second resistor  206 . The operational amplifier  306 , which is configured as a unity gain buffer, is adjusted so that it drives the center point (CP) between the resistors  204  and  206  to the same potential as the second lead L 2  (first node). As a result, there is zero voltage drop across the first resistor  204 , which means that no current will flow through the first resistor  204  nor through the second resistor  206 . This means that all of the current from the circuit tester  208  will attempt to flow through the first and second gap layers  76  and  78 . With this arrangement the first and second resistors  204  and  206  do not need to be equal. 
     Assuming that the circuit tester  208  applies 2 volts between the second lead L 2  and the shield layers S 1  and S 2 , the potential of the node at the center point (CP) will also rise to 2 volts. It is therefore helpful to make the second resistor  206  large enough so that it does not dissipate an excessive amount of power which could cause the second resistor  206  to melt. The value of the first resistor  204  is preferably smaller than the resistance of the second resistor  206  so that the series resistance R S1  plus R S2  is made as low as possible. It should be noted that the circuit tester  208  and the resistances  204  and  206  can optionally be connected to the first lead L 1  instead of the second lead L 2  in which instance the resistances R G1  and R G2  will be the resistances of the first and second gap layers  76  and  78  between the first lead layer L 1  and the first and second shield layers S 1  and S 2 . It should further be noted that in either instance that the embodiment shown in  FIG. 14  does not enable a determination of the resistances R G1  and R G2  of the first and second gap layers  76  and  78  separately but, in contrast, measures these resistances in parallel, which parallel reading excludes the resistances R S1  and R S2  of the resistors  204  and  206  because of the operation of the operational amplifier  306 . 
     Third Embodiment of the Invention 
       FIG. 15  shows a third embodiment  400  of the present invention which can measure the resistances R G1  and R G2  of the first and second gap layers  76  and  78  separately and is therefore an improvement over the embodiment  300  in FIG.  14 . The embodiment  400  is the same as the embodiment  300  in  FIG. 14  except for the following. The first and second shield layers S 1  and S 2  are no longer shorted together and a third resistor  402  having a resistance R S3  is connected between the center point (second node) and the first shield layer S 1 . The resistances R G1  and R G2  of the first and second gap layers  76  and  78  can now be determined separately. The resistance R G2  of the second gap layer  78  can be determined when the circuit tester  208  is connected across the second shield layer S 2  and the second lead layer L 2 . Optionally, the resistance R G1  of the first read gap layer  76  can be determined by connecting the circuit tester  208  across the first shield layer S 1  and the second lead layer L 2 , as shown in FIG.  16 . Again, it should be understood that since there is no current through the first resistor  204  because of the operational amplifier  306  there is no current through either of the resistors  206  and  402 . Further, in either of the arrangements in  FIGS. 15 and 16 , the first lead L 1  is floating. It should be further understood that all of the connections can be made between the first lead L 1  instead of the second lead L 2 , as discussed hereinabove. 
     A Method of Making 
     Another aspect of the present invention includes a method of making all of the aforementioned components. A still further aspect of the invention includes simultaneously patterning a sensor material layer for forming the sensor  74  and the resistors  204  and  206  or the resistors  204 ,  206  and  402 . This may be accomplished by first depositing multiple films of the sensor  74  on a wafer, such as a wafer  500  in FIG.  17 . The sensor material layer may then be patterned by a positive photoresist which covers the MR sensor and the resistors which are to be retained. Ion milling then removes all of the sensor material layer except that which is covered. The degree of covering the resistors determines their resistances. This then enables the MR sensor and the resistors to be simultaneously formed, thereby saving fabrication steps. It should be noted that when this method is employed that the sensor  74  and the resistors  204  and  206  or the resistors  204 ,  206  and  402  will be coplanar.  FIG. 17  shows rows and columns of magnetic heads  502  formed thereon. After completion of the magnetic heads  502  the wafer is diced into rows of magnetic heads and the rows are lapped to form the air bearing surface. 
       FIG. 18  shows an exemplary plan layout  700  of the embodiments shown in  FIGS. 12 ,  13  and  14 . Lead layers  134  and  136  are shown connected to the sensor  74  and first and second lead layer extensions  150  and  152  interconnect the first and second lead layers  134  and  136  to the pads  104  and  106 , shown on the slider in  FIG. 2 , via first and second studs (not shown). The second lead layer  136  is connected to the first and second resistors  204  and  206  and the second resistor  206  is connected to the first shield layer  80  (see  FIG. 14 ) by a via  708 . A pad  710 , which is shown in phantom, is located at the surface of the slider and is interconnected to the center point (CP) between the resistors by one or more vias at  712 . The first shield layer  80 , shown in phantom, is located below the sensor  74  and the first and second lead layers  134  and  136  and is separated therefrom by the first read gap layer  76 . After the second gap layer  78  is deposited a via  714  is formed down to the first shield layer  80  so that when the second shield layer  82  is deposited on top of the second read gap layer  78  the first and second shield layers are interconnected. During subsequent fabrication of the head a stud is provided between the via  714  and a pad (not shown) at the surface of the slider. In practice the operational amplifier  306  is interconnected to the pad  710  and the circuit tester  208  is interconnected between the second lead layer extension  152  and the pad to the via  714 . As discussed hereinabove, the sensor  74  and the first and second resistors  204  and  206  may be deposited simultaneously and patterned simultaneously. Alternatively, the sensor may be deposited and patterned separately and the first and second resistors  204  and  206  may be deposited simultaneously and patterned simultaneously. It should be understood that vias are simply holes in the structure that are filled with a conductive material such as copper. It should further be understood that in the embodiment shown in  FIG. 18  that after lapping a row of magnetic heads all of the structure below an air bearing surface (ABS) of the sensor is removed. Alternatively, this structure may be on an opposite side of the sensor in which case one or more delete pads at the surface of the slider may be severed by a laser beam to disconnect critical portions of the test circuitry from the sensor. 
     Clearly, other embodiments and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.