Thin film magnetic recording heads are used to read data from and write data to magnetic data storage media, typically magnetic data storage disks. As the amount of data stored in magnetic data storage disks has increased, the dimensions of the components in magnetic heads have decreased. The data are stored as minute magnetic domains that are arranged in a spiral track on the disk. With data stored in this density, it is necessary to accurately fabricate a magnetic head with very small components.
As general background, FIG. 1 shows a side view of a slider 10 containing a magnetic head 104. Slider 10 is “flying” over a magnetic data storage disk 12, which is rotating rapidly (e.g., at 7200 rpm) in the direction indicated by the arrow. Slider 10 has a leading edge 102 and a trailing edge 104. As disk 12 rotates, a cushion of air supports slider 10 at a flying height H above disk 12. In order to read and write the densely packed data on disk 12, the flying height H must be very low (e.g., 10 nm) and the tiny components of magnetic head 104 must be fabricated with great precision.
The surface of slider 10 facing disk 12 is commonly referred to as the air-bearing surface (ABS). FIG. 2 is a plan view of a portion of the ABS of magnetic head 104, viewed from below. The portion of the ABS shown in FIG. 2 contains two poles, designated P1 and P2 that are used to write data to disk 12. Poles P1 and P2 can be used exclusively to write data to disk 12, and a separate structure can be used to read data from disk 12. Alternatively, poles P1 and P2 can be used to read data from and write data to disk 12. The arrow indicates the direction of disk 12 relative to head 104. As shown, bottom pole P1 is formed of two layers 214 and 216 and is separated from top pole P2 by a non magnetic gap layer 226. Poles P1 and P2 are formed of a magnetic metal or alloy such as NiFe, CoFe or CoNiFe. Gap layer 226 is formed of a nonmagnetic material, typically alumina (Al2O3).
The smaller dimension of top pole P2 is referred to as the critical dimension (CD). The CD corresponds roughly to the width of a data track of disk 12. A magnetic flux extends outward from the ABS of magnetic head 104 into disk 12, and this flux is used to write data to disk 12. The CD of bottom pole P1 determines the width of this magnetic flux. At very high recording densities, it is important that the CD of top pole P2 be formed very accurately and the sides of top pole P2 be highly planar and parallel.
As those skilled in the art will understand, magnetic head 104 is fabricated by depositing a succession of layers on a wafer or substrate. The layers are deposited one on top of another in the direction from left to right in FIG. 2. A large number of magnetic heads are formed on a single wafer and are separated from each other after the fabrication process has been completed.
FIG. 3 is a cross-sectional view of magnetic head 104 taken through top pole P2 at the section designated 3—3 in FIG. 2. Note that, in FIG. 3, magnetic head 104 has been rotated 90° as compared with FIG. 1, so that the ABS faces the right side of the drawing.
The write structure of magnetic head 104 will now be described. Starting at the bottom, in direct contact with a substrate 200 is an undercoat layer 210, which is typically made of Al2O3. Above layer 212 are layers 214 and 216 which together form the bottom pole P1 of magnetic head 204. A coil C1 is formed in an opening in layer 216, separated from layer 214 by an insulating layer 218. Layer 220 forms top pole P2. Layer 222, normally referred to as the yoke, is formed of a magnetic material such as NiFe. It is important that the yoke layer 222 be in magnetic contact with layer 220. Layer 222 is curved, and a coil C2 is formed in the space created by the curve in layer 222. Poles P1 and P2 are separated by an insulating layer 224 which becomes gap layer 226 at the air-bearing surface ABS. To write data, a current is applied through terminals (not shown) that connect to coils C1 and C2. This current induces a magnetic field across gap layer 226, which writes data onto a magnetic data storage disk.
An overcoat layer 206 covering yoke layer 222 is typically made of Al2O3.
Methods of fabricating bottom pole P1 and coils C1 and C2 and the intervening insulating layers are well known in the art and will not be described here.
FIGS. 4A and 4B illustrate two steps of a current process for forming top pole P2. As described in greater detail below, top pole P2 is normally formed by plating a layer of a magnetic material (e.g., NiFe, CoFe or CoNiFe) on top of gap layer 226, which is normally made of Al2O3. Initially, a seed layer is formed on top of gap layer 226. The shape of top pole P2 is defined by a trench that is formed in a photoresist layer (not shown in FIGS. 4A and 4B) before the plating process begins.
The magnetic layer that forms top pole P2 is plated and then ion milled to a thickness of about 2.3 μm, as shown in FIG. 4A. Then a layer of Al2O3 is deposited to a thickness of about 4.2 μm. The final thickness or height of top pole P2 (represented by the vertical dimension in FIGS. 4A and 4B.) is about 1.5 μm. To achieve this thickness, a chemical-mechanical polishing (CMP) process is used to remove a sufficient amount of the Al2O3 layer and top pole P2 until the structure shown in FIG. 4B is obtained. In addition to providing the desired thickness of top pole P2, this process assures that all top poles P2 in the wafer will be opened so that the connection to yoke layer 222 can be made, as described above.
There are several problems with this process. First, an extra thickness of approximately 0.8 μm of top pole P2 and approximately 2.7 μm of Al2O3 must be deposited and removed by CMP, consuming material and valuable processing time. Second, as described below, the photoresist layer that is used to define the shape of top pole P2 must be thicker to form a top pole P2 with an initial thickness of 3.8 μm. Correspondingly, the aspect ratio (depth divided by width) of the trench that is used to form the top pole P2 must be greater. This in turn makes it more difficult to form a trench with straight walls and predictable critical dimension (CD), particularly as shorter wavelength radiation is used to expose and pattern the photoresist layer. Generally, as the CD decreases in heads that are used to read and write densely packed data, the industry is transitioning from I-line photolithography (λ=365 nm) to deep ultraviolet (DUV) photolithography (λ=248 nm) to form the trench that is used to define the top pole P2. DUV photolithography provides better definition and hence control of the CD than I-line photolithography, but DUV radiation has a smaller depth of focus than I-line radiation. This makes it more difficult to work with a relatively thick photoresist layer.
Moreover, as shown in FIG. 5A, the opening of a trench formed with DUV photolithography has a characteristic “flare,” which produces a top pole 50 of the kind shown in FIG. 5B. As those skilled in the art will understand, top pole 50 casts a “shadow” that increases the difficulty of subsequently removing material by isotropic processes such as ion milling. Furthermore, the “flare” increases the difficulty of controlling the profile of the walls of the top pole itself during subsequent ion milling processes. Therefore, to ensure that top pole P2 is formed in the “straight” portion of the trench, the photoresist layer used with DUV photolithography needs to be at least 2 μm deeper than the thickness of the as-plated top pole P2. This in turn significantly degrades the resolution of the DUV photolithography and the control of the profile of the top pole P2.
Finally, the CMP process that is used to define the thickness of the top pole P2 is inherently difficult to control, and this leads to variation (defined as sigma) in the thickness of the top poles P2 within a given wafer (WIW) and from wafer to wafer (WTW).
Accordingly, there is a need for a process that avoids these problems in the formation of the top pole P2 using DUV photolithography.