Patent Publication Number: US-6219207-B1

Title: Read sensor having high conductivity multilayer lead structure with a molybdenum layer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high conductivity multilayer lead structure with a molybdenum layer for a read sensor and, more particularly, to a lead layer structure that has a molybdenum layer on a seed layer structure which provides conductivity nearly equivalent to a tantalum (Ta) and gold (Au) multilayer lead structure. 
     2. Description of the Related Art 
     The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm that supports the slider above the rotating disk and an actuator 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 impressions 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. 
     The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a nonmagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field or the lack thereof writes information in tracks on moving media, such as in circular tracks on a rotating disk. 
     The read head includes a sensor that is located between nonmagnetic nonconductive first and second read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to an air bearing surface (ABS) of the head and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer. 
     The spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. MR coefficient is dr/R were dr is the change in resistance of the spin valve sensor in response to signal fields and R is the resistance of the spin valve sensor before the change in resistance. 
     The read sensor, which may be a spin valve sensor or an AMR sensor, is bounded by a first edge at the ABS, a recessed edge spaced from the ABS and first and second side edges. First and second lead layers are connected to the first and second side edges respectively of the read sensor for the purpose of conducting the sense current therethrough. This type of connection is known in the art as a contiguous junction. Another type of connection is a continuous junction which is when the first and second lead layers overlap side portions of the read sensor layers with a space therebetween that defines the track width of the read head. It is important that the lead layers have low conductivity and be capable of withstanding operating temperatures in a magnetic disk drive which may be 80° C.-120° C. Low conductivity is important for minimizing heat and noise generated by the lead layers. If the lead layers have a high resistance they must be made thicker in order to reduce the heat and noise produced by the lead layers. Unfortunately, thicker lead layers increase the profile of the read head on each side of the sensor which is replicated in subsequent layers all the way to the write gap layer in the write head. This can cause write gap curvature which will cause the write head to write curved magnetic impressions into circular tracks on a rotating magnetic disk. This is undesirable since the read head reads straight across and will lose a portion of the signal at the outer edges of the magnetic impressions. 
     A typical material employed for lead layers is tantalum (Ta). While tantalum (Ta) operates well at operating temperatures it has a high resistance and suffers from the disadvantages described hereinabove. The resistance of a typical tantalum (Ta) lead layer can range from 2.2 to 2.8 ohms/sq. Gold (Au) is a desirable substitute for tantalum (Ta) because gold has a low resistance. A typical gold/nickel (Au/Ni) or gold/tantalum (Au/Ta) multilayer can have a resistance of about 1.0 ohms/sq. While such multilayers are very desirable from the standpoint of their conductivity, gold (Au) has not been able to withstand the aforementioned operating temperatures of the read head. The lead layers have edges at the ABS in the same manner as the layers that make up the read sensor. At operating temperatures the heat and attendant stresses cause the gold to ooze at the ABS like toothpaste. A contributing factor may be the expansion of layers in the read head which are adjacent the lead layers. Oozing of the gold at the ABS degrades the performance of the lead layers and can short the lead layers to the first and second shield layers, as well as shorting across edges of sensitive layers of the read sensor. Accordingly, there is a strong-felt need to provide lead layers that have a low resistance similar to gold but hard enough to withstand operating temperatures of the read head. 
     SUMMARY OF THE INVENTION 
     The lead layers of the present invention include a layer of molybdenum (Mo) and a seed layer structure. The seed layer structure includes a layer of tantalum (Ta) and/or a layer of chromium (Cr). In a preferred embodiment a seed layer of chromium (Cr) is located between a seed layer of tantalum (Ta) and the layer of molybdenum (Mo). The resistance of a lead layer with such a structure can range from 1.14 to 1.45 ohms/sq. The 1.14 resistance value is nearly as low as the 1.0 resistance value for the aforementioned tantalum (Ta) and gold (Au) multilayer lead structure of the same thickness. Further, the present molybdenum (Mo) layer and seed layer structure can easily withstand the operating temperatures of the read head. Since molybdenum (Mo) is a harder material than gold (Au) there is no oozing of the molybdenum at the ABS. 
     An object of the present invention is to provide lead layers for a read sensor which have low resistance and reliability at operating temperatures of the read head. 
     A further object is to provide a low resistance seed layer structure for a molybdenum (Mo) layer in a lead layer structure which increases the conductivity of the molybdenum (Mo) layer. 
     Other objects and advantages of the invention will become apparent 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 ; 
     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 isometric ABS illustration of a read head; 
     FIG. 12 is an ABS illustration of a first example of first and second lead layer structures employing multilayers of gold (Au) and tantalum (Ta); 
     FIG. 13 is an ABS illustration of a second example of first and second lead layer structures employing tantalum (Ta); 
     FIG. 14 is an ABS illustration of a third example of first and second lead layer structures employing tantalum (Ta); 
     FIG. 15 is an ABS illustration of a fourth example and first embodiment of the invention of first and second lead layer structures employing molybdenum (Mo) on a chromium (Cr) seed layer; 
     FIG. 16 is an ABS illustration of a fifth example and second embodiment of the invention of first and second lead layer structures employing molybdenum (Mo) on a tantalum (Ta) seed layer; 
     FIG. 17 is an ABS illustration of a sixth example and third embodiment of the present invention of first and second lead layer structures employing a molybdenum (Mo) layer on a seed layer structure including a first layer of tantalum (Ta) and a second layer of chromium (Cr); and 
     FIG. 18 is an ABS illustration of a seventh example and fourth embodiment of the present invention of first and second lead layer structures showing a reduced thickness of the molybdenum (Mo) layer on a seed layer structure including a first seed layer of tantalum (Ta) and a second seed layer of chromium (Cr). 
    
    
     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 motor  36  that is controlled by a motor controller  38 . A slider  42  has a combined read and write magnetic head assembly  40  and is supported by a suspension  44  and actuator arm  46 . 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  position the slider  42  so that the magnetic head  40  assembly is in a transducing relationship with a surface of the magnetic disk  34 . When the disk  34  is rotated by the 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 assembly  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 assembly  40 , provides motor drive signals for rotating the magnetic disk  34 , and provides control signals for moving the slider to various tracks. In FIG. 4 the slider  42  is shown mounted to the suspension  44 . The components described hereinabove may be mounted on a frame  54  of a housing, as shown in FIG.  3 . 
     FIG. 5 is an ABS view of the slider  42  and the head  40 . The slider has a center rail  56  that supports the magnetic head assembly  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 assembly  40  is at a trailing edge  66  of the slider. 
     FIG. 6 is a side cross-sectional elevation view of a piggyback magnetic head assembly  40 , which includes a write head portion  70  and a read head portion  72 , the read head portion employing an exemplary spin valve sensor  74 . FIG. 8 is an ABS view of FIG.  6 . The spin valve sensor  74  is sandwiched between nonmagnetic nonconductive 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 spin valve 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 assembly  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 spin valve 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. 8) 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 an exemplary spin valve sensor  130  which is located on an antiferromagnetic (AFM) pinning layer  132 . A ferromagnetic pinned layer in the spin valve sensor  130  is pinned by the magnetic spins of the pinning layer  132 . The AFM pinning layer may be 425 Å of nickel oxide (NiO). First and second hard bias and lead layers  134  and  136  are connected to first and second side edges  138  and  140  of the spin valve 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. Optionally, the connection may be a continuous junction in which first and second hard bias and lead layers extend over and are coupled to first and second side portions of spin valve sensor layers on each side of sensor portions of these layers. 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 spin valve sensor  130  for stabilizing the magnetic domains therein. The AFM pinning layer  132 , the spin valve sensor  130  and the first and second hard bias and lead layers  134  and  136  are located between nonmagnetic nonconductive 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 . 
     EXAMPLE 1 
     FIG. 12 shows an ABS illustration of a first example  200  of first and second lead layer structures  202  and  204 . The lead layer structures  202  and  204  are electrically connected to first and second side edges  206  and  208  of a read sensor  210  which may be a spin valve sensor or an AMR sensor. The read sensor  210  is further bounded by its edge at the ABS, which is in the plane of the paper in FIG. 12, and a recessed edge, which is shown in FIGS. 6 and 8. Each lead layer structure  202  and  204  includes first, second and third gold (Au) layers  212 ,  214  and  216 , which are interlayered with first, second, third and fourth tantalum (Ta) layers  218 ,  220 ,  222  and  224 . The tantalum (Ta) layers are 50 Å thick and the gold (Au) layers are 200 Å thick. 
     In a typical read head employing a spin valve sensor, first and second hard bias layer structures for magnetically biasing the free layer of the spin valve sensor parallel to the ABS along its length between the first and second side edges  206  and  208 . Each hard bias layer structure includes a cobalt platinum chromium (CoPtCr) layer  230  on a chromium (Cr) seed layer  232 . The biasing is necessary for stabilizing the operation of the free layer. The cobalt platinum chromium (CoPtCr) layer  230  is 200 Å thick and the chromium (Cr) seed layer  232  is 35 Å thick. The first and second lead layer structures  202  and  204  are typically located on the first and second hard bias layer structures  226  and  228 . The sensor  200 , the first and second lead layer structures  202  and  204  and the first and second hard bias layer structures are located between first and second read gap layers, the first read gap layer (G 1 )  234  being shown in FIG.  12 . The first and second gap layers (G 1 ) and (G 2 ) are located between first and second shield layers (S 1 ) and (S 2 ), as shown in FIG.  11 . 
     The lead layer structures  202  and  204  in FIG. 12 have a very low resistance which is desirable for the purpose of reducing heat and noise generated within the read head. The resistance of each of the lead layers  202  and  204  is about 1.0 ohms/sq. Unfortunately the gold (Au) layers  212 ,  214  and  216  fail structurally during operating temperatures of the read head which may be 80° C.-120° C. At these temperatures and attendant stresses within the read head the gold oozes from the lead layer structures  202  and  204  at the ABS like toothpaste. This seriously degrades the performance of the lead layer structures  202  and  204  and can short the lead layer structures to the first and second shield layers at the ABS or short across edges of sensitive layers of the sensor at the ABS. Accordingly, I undertook a search for a lead layer structure which has low resistance comparable to the tantalum (Ta) and gold (Au) multilayer lead structure but with increased hardness and reliability to prevent oozing at the ABS. 
     EXAMPLE 2 
     FIG. 13 shows an ABS illustration of a second example  300  of first and second lead layer structures  302  and  304 . The illustration in FIG. 13 is the same as the illustration in FIG. 12 except for the first and second lead layer structures  302  and  304 . Each of the lead layer structures  302  and  304  includes a layer of tantalum (Ta)  306  on a multi seed layer structure which includes a first seed layer of tantalum (Ta)  308  and a second seed layer of chromium (Cr)  310 . The tantalum (Ta) layer  306  is 750 Å thick and each of the seed layers  308  and  310  is 35 Å thick. The resistance of each of the lead layer structures  302  and  304  was 2.2 ohms/sq. which is more than twice the resistance of the gold lead layer structures  202  and  204  in FIG.  12 . This high resistance is undesirable. 
     EXAMPLE 3 
     FIG. 14 is an ABS illustration of a third example  400  of the first and second lead layer structures  402  and  404 . The lead layer structures  402  and  404  are the same as the lead layer structures  302  and  304  in FIG. 13 except for the tantalum layers  406  which are 600 Å thick instead of 750 Å thick. The resistance of the first and second lead layer structures  402  and  404  in FIG. 14 was 2.8 ohms/sq. This is almost three times the resistance of the first and second gold lead layer structures  202  and  204  in FIG.  12 . 
     EXAMPLE 4 
     First Embodiment of the Invention 
     FIG. 15 shows an ABS illustration of a fourth example  500  and first embodiment of the invention of first and second lead layer structures  502  and  504 . Example  500  is the same as the example  200  shown in FIG. 12 except for the first and second lead layer structures  502  and  504 . Each of the lead layer structures  502  and  504  includes a molybdenum (Mo) layer  506  on a chromium (Cr) seed layer  508 . The molybdenum (Mo) layer  506  is 750 Å thick and the chromium (Cr) layer  508  is 35 Å thick. Molybdenum (Mo) is a desirable lead layer material because it has low resistance, it is hard and it is reliable at operating temperatures of the read head. The resistance of each of the first and second lead layer structures  502  and  504  was 1.7 ohms/sq. This is an improvement over the lead layer structures shown in FIGS. 13 and 14. 
     EXAMPLE 5 
     Second Embodiment of the Invention 
     FIG. 16 is an ABS illustration of a fifth example  600  and second embodiment of the invention of first and second lead layer structures  602  and  604 . Example  600  is the same as Example  200  in FIG. 12 except for the first and second lead layer structures  602  and  604 . Each of the first and second lead layer structures  602  and  604  includes a molybdenum (Mo) layer  606  on a tantalum (Ta) seed layer  608 . The molybdenum (Mo) layer  606  is 750 Å thick and the tantalum (Ta) layer  608  is 35 Å thick. The only difference between the example  600  in FIG.  16  and the example  500  in FIG. 15 is that a tantalum seed layer  608  is employed instead of a chromium (Cr) seed layer  508 . The resistance of the first and second lead layer structures  602  and  604  was 2.0 ohms/sq. which is an improvement over the lead layer structures shown in FIGS. 13 and 14. 
     EXAMPLE 6 
     Third Embodiment of the Invention 
     FIG. 17 is an ABS illustration of a sixth example  700  and third embodiment of the invention of first and second lead layer structures  702  and  704 . Example  700  is the same as example  200  in FIG. 12 except for the first and second lead layer structures  702  and  704 . Each of the lead layer structures  702  and  704  includes a molybdenum (Mo) layer  706  on a multilayer seed layer structure which includes a first seed layer of tantalum (Ta)  708  and a second seed layer of chromium (Cr)  710 . The second seed layer  710  is located between and interfaces the first seed layer  708  and the molybdenum (Mo) layer  706 . The thickness of the molybdenum (Mo) layer is 750 Å and the thickness of each of the first and second seed layers  708  and  710  is 35 Å. The seed layer structure, including the layers  708  and  710 , produced a surprising increase in conductivity of the molybdenum (Mo)  706 . The resistance of the molybdenum (Mo) layer  706  was lowered by the multi seed layer structure lower than the resistance of molybdenum (Mo) per se. If the molybdenum (Mo) layer  706  is directly on the hard bias layer  230  of cobalt platinum chromium (CoPtCr) without the seed layers  708  and  710  the resistance is 1.69 ohms/sq. With the seed layers  708  and  710 , the resistance of each of the first and second lead layer structures  702  and  704  was 1.14 ohms/sq. This resistance is comparable to the resistance of 1.0 for the tantalum (Ta) and gold (Au) multilayer lead structure shown in FIG.  12 . Accordingly, the lead layer structures  702  and  704  are a preferred embodiment providing the desired conductivity, hardness and reliability at operating temperatures of the read head. 
     EXAMPLE 7 
     Fourth Embodiment of the Invention 
     FIG. 18 is an ABS illustration of a seventh example  800  and fourth embodiment of the invention of first and second lead layer structures  802  and  804 . The lead layer structures  802  and  804  are the same as the lead layer structures  702  and  704  in FIG. 17 except for molybdenum (Mo) layers  806  which are 600 Å thick instead of 750 Å thick. The resistance of each of the lead layer structures  802  and  804  was 1.45 ohms/sq. The increase in resistance is due to the reduction in thickness of the molybdenum layers  806 . However, the resistance of 1.45 is significantly less than the resistance of the lead layer structures in FIGS. 13 and 14. 
     OBSERVATIONS 
     Testing of the molybdenum layers in examples shown in FIGS. 17 and 18 show these layers to have a superior texture which promotes their low resistance. The grain size of the molybdenum (Mo) layers  706  in FIG. 17 is about 390 Å as compared to 190 Å without the tantalum (Ta) and chromium (Cr) seed layers  708  and  710  in FIG.  17 . The molybdenum (Mo) layers in FIG. 17 have a very strong 110 texture as compared to molybdenum (Mo) without the seed layers  708  and  710 . It is believed that the multilayer seed layer structure contributed to a desirable texture of the molybdenum layers when they were formed on the seed layer structures. The lead layer structures may be employed for any type of read sensor including spin valve and AMR sensors and read sensors with either a contiguous junction or a continuous junction. It should be understood that the multilayered seed layer structure may include additional tantalum (Ta) and chromium (Cr) layers without departing from the spirit of the invention and may be employed as capping layers instead of seed layers. Further, the hard bias layer structures may include different materials and additional layers. 
     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.