Patent Application
20060273066
The present invention relates to a wafer processing technique and more particularly to a method for manufacturing a device, such as a magnetoresistive sensor or other device, having a very narrow width and small width variation.
The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic 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 read and write heads are directly located on a slider that has an air bearing surface (ABS). 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. 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 gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been 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 the air bearing surface (ABS) 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 thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos Θ, where Θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP 1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
The push for ever increased data rate and data capacity results in a need for GMR sensors having ever smaller track width. This smaller track width determines the number of tracks of data that can be fit onto a given magnetic medium. The track width is, however, limited by manufacturing capabilities. For example, the width of magnetoresisive sensor has been limited to a width at which a mask, such as photoresist, can be accurately photolithographically patterned. Optical lithography is running out of the resolution to provide narrow read track width sensors with tight sigma control.
Techniques such as phase shifting masks and E-beam lithography have provided some additional resolution and corresponding decrease in track width, but are expensive and limited to trackwidths of about 50 nm. To appreciate the problem, consider that the sigma of a photolithographic process is about 5 nm. This sigma does not decrease with decreasing track width, so it can be appreciated that trackwidths approaching 5 nm are impractical using such photolithographic techniques.
Therefore, a strong felt need exists for a wafer manufacturing process that can form structures having very small track widths with precise width control. Preferably, such a method would be able to build devices such as magnetoresistive heads having widths less than 10 nm with sigma of about 1 nm. Such a method would also not require the use of expensive tooling such as E-beam lithography tooling or phase shifting masks.
The present invention provides a method for constructing a device such as a magnetoresistive sensor having an extremely narrow width and tight distribution (small sigma of the width). A plurality of sensor layers is deposited on a substrate. A first mask, formed of for example photoresist, is formed over the sensor layers. The first mask is formed to cover only a portion of the sensor layers, the first mask terminating at an edge to leave a portion of the sensor layers uncovered. A layer of material that is susceptible to removal by reactive ion etching (RIEable material) is then deposited by a conformal deposition method so that it covers the edge of the first mask as well as the top and on the sides of the first mask and exposed portion of the sensor layers. A reactive ion etch (RIE) is then performed to remove the RIEable material from the top of the first mask and from the uncovered portion of the sensor layers leaving a portion of the RIEable material at the edge of the first mask. The first mask can then be chemically stripped off leaving the remaining RIEable material as a very thin second mask for defining the device (sensor) by a subsequent ion milling operation.
The invention advantageously allows devices such as sensor to be constructed with extremely narrow widths while providing extremely fine control over the width distribution. The invention can advantageously achieve this end, without the need for expensive E-beam lithography or phase shifting photolithographic masks.
The present invention takes advantage of both the directional nature of reactive ion etching and the selectivity of the RIEable material and isotropic deposition. The RIEable material, which can be for example SiO2, Al2O3, SiN4 or SiOxNy, W, Ta, has the property that it is removed by RIE much more quickly than the other materials such as the sensor layers. In fact this selectivity ratio is about 50/1.
The invention also takes advantage of the directional nature of RIE. This causes the RIE to remove the horizontally disposed portions of the RIEable material at a much faster rate than the vertically disposed portion formed on the side wall of the first mask. This allows the RIE to leave the vertically disposed portion as a very thin mask while removing the other portions.
Taking advantage of these features (the selectivity of the RIEable material and the directional nature of RIE) allows a very narrow mask to be formed without generating an asymmetry in the underlying sensor layers. Without the advantages of selectivity and directionality, the process of forming the narrow mask would also result in removal of sensor material at only one side of the mask, causing a sever shape asymmetry in the produced sensor device.
These and other features and advantages of the invention will be apparent upon reading of the following detailed description of preferred embodiments taken in conjunction with the Figures in which like reference numerals indicate like elements throughout.
For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale.
FIGS. 3 is an ABS view of a mangeotresistive sensor taken from circle 3 of
The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein.
Referring now to
At least one slider 113 is positioned near the magnetic disk 112, each slider 113 supporting one or more magnetic head assemblies 121. As the magnetic disk rotates, slider 113 moves radially in and out over the disk surface 122 so that the magnetic head assembly 121 may access different tracks of the magnetic disk where desired data are written. Each slider 113 is attached to an actuator arm 119 by way of a suspension 115. The suspension 115 provides a slight spring force which biases slider 113 against the disk surface 122. Each actuator arm 119 is attached to an actuator means 127. The actuator means 127 as shown in
During operation of the disk storage system, the rotation of the magnetic disk 112 generates an air bearing between the slider 113 and the disk surface 122 which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension 115 and supports slider 113 off and slightly above the disk surface by a small, substantially constant spacing during normal operation.
The various components of the disk storage system are controlled in operation by control signals generated by control unit 129, such as access control signals and internal clock signals. Typically, the control unit 129 comprises logic control circuits, storage means and a microprocessor. The control unit 129 generates control signals to control various system operations such as drive motor control signals on line 123 and head position and seek control signals on line 128. The control signals on line 128 provide the desired current profiles to optimally move and position slider 113 to the desired data track on disk 112. Write and read signals are communicated to and from write and read heads 121 by way of recording channel 125.
With reference to
With reference now to
The free layer can be constructed of Co, CoFe, NiFe or some combination of these or other materials. The free layer has a magnetic moment 326 that is biased in a desired direction parallel with the ABS, but that is free to rotate in response to a magnetic field, such as from a magnetic medium. The free layer moment 326 is biased by a first and second hard bias layers 328, 330, constructed of a hard magnetic material such as, for example, CoPtCr. First and second electrically conductive leads 332, 334, formed over the hard bias layers 328, 330 conduct electrical sense current to the sensor stack 302.
The sensor 300 may also include a seed layer 336 formed at the bottom of the sensor stack to promote a desired epitaxial grain growth in the subsequently deposited layers. A capping layer 338, such as Ta, may be provided at the top of the sensor stack 302 to protect the other sensor layers from damage during manufacture. The track width (TW) 340 of the sensor 300 is the width of the sensor stack, and more accurately, is the width of the free layer 312 as viewed from the ABS.
With reference now to
With reference now to
With reference now to
The above described deposition methods of the materials described for layer 502 provide extremely good control of thickness and uniformity. As will be seen below, this is critical in defining and controlling width of the finished device (track width TW 340 of the sensor 300). With reference now to
The present invention takes great advantage of (1) the large selective removal rate of the layer 408 as compared with the device layers 404 during the RIE process, and (2) the strongly directional nature of RIE compared with other material removal processes such as ion milling. This allows the horizontal components of the layer 408 to be readily removed, while leaving a vertical portion 408. More importantly this allows the horizontal portions of the layer 408 to be removed while leaving the device layers 404 (sensor layers 302) virtually unaffected by the RIE. As a result, the device layer 404 has the same thickness after RIE on both sides of the narrow mask 604 (ie, same thickness under the photoresist 406 as in the exposed region at the right in
If other non-selectively removed materials were used and if another material removal process other than the RIE process described above were used, the process would not be practical. For example, if the layer 408 were constructed of an electrically conductive material such as Cu, Pt, Pd, Au, Ag, NiFeX, CoZrY, or FeAISi, and removed by, for example an ion milling operation, a large portion of the underlying device layer 404 would be removed from the side not covered by photoresist 406. This would result in an unacceptable asymmetry in the finished device. What's more, the use of ion milling would result in a large amount of material 404 begin redeposited (redep) on the sides of the mask layer 604. This would require the used of a special angled ion mill to removed the redeposited material. Therefore, taking advantage of both the selectivity of the materials described above, along with the directional nature of the RIE process provides an enormous advantage in the present invention.
The RIE 602 results in very negligible redeposition of material. Furthermore, there is only about 1% reduction in thickness of the vertical portion of the layer 408 during the RIE, and this can be predicted and accounted for when depositing the layer 408. What's more the process is scaleable, in that the sigma (variation in thickness of the mask 604) is only about 1.5% of the width of the mask 604 and decreases proportionally with decreased mask thickness. This can be compared with prior art methods described in the Background of the Invention, where the sigma was fixed at about 5 nm so that any significant reduction in device width was completely consumed by the sigma.
If the material layer 408 is constructed of Alumina the RIE can be performed using a Fl or Cl based chemistry (atmosphere in the chamber). Using a Chlorine chemistry gives better selectivity for removing the alumina, but also results in more corrosion of the other layers, whereas a fluorine chemistry results in less corrosion but also less selectivity. If the layer 408 is constructed of one of the other described materials, the RIE can be performed in a fluorine chemistry. In such case, the RIE 602 can be performed in an atmosphere that includes CHF4, CHF3, or CF3.
With reference now to
With reference now to
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
