Abstract:
The invention teaches how to image and etch very narrow and isolated features without resorting to expensive OPC measures. Assist features are placed close to the main feature so that the local pattern density becomes semi-dense or dense and the pattern is imaged in a bilayer suitable for use in liftoff. The lower (easily etched) layer is then exposed to a suitable solvent so that the upper (etch resistant) layer is slowly undercut. Undercutting can be terminated as long as all the assist features have been lifted off. Although the original isolated feature will, in most cases, also have all of its lower layer removed, it does not lift off because, as a requirement of the process, at least one of its ends remains connected to an area of photoresist that is too wide to be fully undercut.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to the general field of photolithography with particular reference to reproduction of isolated features.  
         BACKGROUND OF THE INVENTION  
         [0002]    The photolithographic process for the GPC (Giant magnetoresistance Permanent magnet Conductor) layer requires high resolution (smallest dimension) and the tightest control for CD (critical dimension) uniformity. With smaller and smaller design rules being introduced for the track width, a large process window or depth of focus is needed. Several resolution enhancement techniques can be used to widen the process window but each of these has its own limitations or is unsuitable for thin film head applications. For example, the cost of modifying masks by adding sub-resolution features generated by OPC (optical-proximity-correction) software is substantial.  
           [0003]    As another example, off-axis illumination has been proven to improve both resolution as well as depth of focus but works well only in areas where the local pattern density (fraction of the area that is opaque) can be considered to be at least semi-dense (generally about 20% opaque).  
           [0004]    The thin film head industry faces a unique situation in that there is often a need to form an etch mask containing a single isolated line in order to be able to etch through the topmost layers of a read head (typically a Giant Magneto Resistance, or GMR, stack). The present invention discloses how such a mask may be formed without the need to use expensive OPC techniques to assist in the formation of said mask, including said single isolated line.  
           [0005]    A routine search of the prior art was performed with the following references of interest being found:  
           [0006]    Mansfield et al. describe assist features for photolithographs in U.S. Pat. No. 6,421,820. In U.S. Pat. No. 6,303,252, Lin discloses assist features between! semi-dense lines. U.S. Pat. No. 6,165,693 (Lin et al) shows a method of designing assist features. None of these patents are in the magnetic recording field. U.S. Pat. No. 5,491,600 (Chen et al) is an example of a patent in the magnetic recording field showing a PMGI layer for lift-off of a photoresist.  
         SUMMARY OF THE INVENTION  
         [0007]    It has been an object of at least one embodiment of the present invention to provide a process for manufacturing a MR read head, particularly the topmost, or GPC, layer.  
           [0008]    Another object of at least one embodiment of the present invention has been to be able to image and etch very narrow, but isolated, features such as line segments.  
           [0009]    Still another object of at least one embodiment of the present invention has been that the process used for imaging said narrow isolated features require no significant changes to the processes already being used for this purpose.  
           [0010]    A further object of at least one embodiment of the present invention has been that said process not require the use of expensive, time consuming, OPC methods.  
           [0011]    These objects have been achieved by temporarily increasing the pattern density in the immediate vicinity of the isolated feature through the addition of a number of dummy, or assist, features to the pattern. The etch mask, including these modifications is then implemented in a bilayer suitable for later use as a liftoff medium. The lower (easily etched) layer of said bilayer is then exposed to a suitable solvent (such as the developer itself) so that the upper (etch resistant) layer is slowly undercut. This undercutting can be terminated once all the assist features have been lifted off.  
           [0012]    Although the original isolated feature will, in most cases, also have all of its lower layer removed, it does not lift off because, as a requirement of the process, at least one of its ends remains connected to an area of photoresist that is too wide to be fully undercut. The size of the undercut may be adjusted according to process requirements, providing the anchor area, which holds the original feature, does not lift off. Once selective removal of the assist features has been effected, the mask may be used for etching in the usual way by using one of several available processes in which etching occurs only in a direction normal, or close to normal, to the surface.  
           [0013]    The invention is compatible with current MR head manufacturing processes and no special masks are needed. The design of the assist features can be optimized according to the available illumination tools, making the cost of the mask much lower than if OPC had been used together with PSM techniques. The number of masks needed is also less compared to that of conventional isolated-line design. The process window of the GPC layer is thus increased significantly.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 illustrates the mask used to implement the present invention.  
         [0015]    [0015]FIG. 2 is a cross-section through part of FIG. 1  
         [0016]    [0016]FIG. 3 shows the mask of FIG. 1 after selective removal of certain portions thereof.  
         [0017]    [0017]FIG. 4 is a cross-section through part of FIG. 3.  
         [0018]    [0018]FIG. 5 is a plan view of the structure following application of the process of the present invention.  
         [0019]    [0019]FIG. 6 is a cross-section through part of FIG. 5 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    We will disclose the present invention by describing a process for forming the GPC portion of a magneto-resistive read head it will, however, be understood by those skilled in the art that the invention is more general than this and can be applied to many situations where a single, very narrow and isolated, feature needs to be formed by an etching process.  
         [0021]    The process is best understood by first seeing the end product that it is intended to produce. This is shown in schematic cross-section in FIG. 6. Seen there is substrate  21  (normally a layer of ferromagnetic material that will serve as a lower shield) on which has been deposited a series of layers having various magnetic properties and thicknesses which, together, constitute GMR stack  22 . Stack  22  has been etched into the shape shown so that longitudinal bias layer(s)  23  can abut its upper, vertical, portion. Conductive lead layer  24  has been deposited thereon but not onto surface  11  which is the portion of the read head that flies over the ABS (air bearing surface).  
         [0022]    Referring now to FIG. 1, we show a plan view of portion of photoresist layer  12  which has been patterned into the shape shown by exposure to actinic radiation in the form of a reduced image projected from a reticle. The pattern carried on the reticle includes two areas  14   a  and  14   b  that are separated from one another by narrow region  11  that defines the track width of the read head. Currently region  11  would be less than about 0.3 microns wide. It is to be noted that narrow region  11  is connected at its ends to the main body of photoresist which is serving as a common enclosure for  14   a  and  14   b.    
         [0023]    In prior art practice, the pattern seen in FIG. 1 would be limited to elements  11 ,  12 ,  14   a  and  14   b . When the width of element  11  approached the resolution limit of the total optical system steps would have had to be taken to cancel out optical proximity effects. As already noted, this can be both expensive and of limited success, particularly because region  11  is an isolated feature so techniques such as off-axis illumination do not work well.  
         [0024]    In order to solve this problem, the present invention temporarily increases the pattern density in the vicinity of feature  11  by adding (to the reticle pattern) additional free-standing elements  15 . These temporary features need have no particular shape (although line features disposed to lie parallel to element  11  are preferred) and their spacing relative to element  11  or to one another is not critical—so long as they increase the pattern density in the immediate vicinity of  11 . In general, they will be resolved by the optical system, appearing in the developed photoresist image, so they are not to be confused with scattering bars that, while present on the reticle, do not appear in the photoresist image.  
         [0025]    [0025]FIG. 2 is a cross-section of FIG. 1 taken through  2 - 2 . Layer  22  represents the topmost layer(s) of the GMR stack as shown in FIG. 6 but before it has been etched into the shape seen in FIG. 6. The key feature illustrated in FIG. 2 is that photoresist layer  12  does not lie directly on  22 , but rather on layer  25 . Layer  25  is of a material that is highly soluble in at least one etchant and that is patternable through development following selective exposure to radiation. This is in contrast to layer  12  which, though also patternable through development following selective exposure to radiation, is resistant to etching, i.e. photoresist. Note that ‘radiation’ in this context could be light having a wavelength between about 1,590 and 3,650 Angstroms or it could be an electron beam.  
         [0026]    Layers  25  and  12  together form a bilayer which is treated as a single layer for the purposes of exposure to radiation and subsequent development. The result is a bilayer etch mask suitable for use in a liftoff process. Typical examples of materials suitable for use in layer  25  include (but are not limited to) PMGI (polymethylglutarimide) while the thickness of layer  25  was typically between about 0.01 and 1 microns. In the case of layer  12 , typical examples of suitable materials include (but are not limited to) D2N (diazoquinone) and PHS (polyhydroxystyrene) while the thickness of layer  12  was typically between about 0.05 and 5 microns.  
         [0027]    Now follows another key feature of the invention. After normal development, etching of layer  25  is allowed to continue so that layer  12  is steadily undercut. This step may be performed using a separate etchant or the latter may be included in the composition of the developer so achieving the required degree of undercutting becomes a matter of controlled over-development. Either way, the consumption from its edges of layer  25  (schematically shown as region  31  in FIG. 3) is allowed to proceed until it has been completely removed from under assist features  15  so that they lift off substrate  22  and are washed away.  
         [0028]    It is important to note that, unless feature  11  was wider than the assist features  15 , it, too, will have had all of layer  25  beneath it removed. However, unlike elements  15 , element  11  is attached to the main body of photoresist  12  at both its ends. So, even after layer  25  has been removed from under it, it remains in position, being suspended, bridge-like, between locations  32   a  and  32   b  of the main body of photoresist, as shown in FIG. 3. This can be seen in the cross-section  4 - 4  shown in FIG. 4 where centrally located photoresist stripe  12  appears to be floating but is actually anchored at points above and below the plane of the figure.  
         [0029]    Despite the fact that it is not always in contact with substrate  22 , photoresist mask  12  can still be effectively employed during an etching procedure, provided etching only occurs in a direction normal, or near normal, to the substrate surface. Examples of etching methods that satisfy this condition include (but are not limited to) Reactive Ion Etching (RIE) and Ion Beam Milling (IBE).  
         [0030]    Thus, once removal of the assist elements  15  has been completed, etching of layer  22  can proceed, resulting in the formation of cavities  54  with the topmost part of layer  22  being given the shape  52  seen, in plan view, in FIG. 5. The depth to which GMR stack  22  is etched is between about 0.01 and 0.1 microns. This is followed by the deposition, in succession, of one or more layers  23  suitable for providing permanent magnetic bias followed by layer  24  of a conductive material suitable for use in a pair of leads that flank the GMR stack, as seen in FIG. 6.  
         [0031]    After layers  23  and  24  have been deposited, the exposure of layer  25  to the solvent is allowed to continue for as long as it takes to achieve complete liftoff of all deposited material that was in contact with photoresist layer  12 , thereby exposing areas  11  and  52  (FIG. 5).  
         [0032]    As already noted, the invention is not limited to magnetic head applications. Thus whenever a very narrow, but isolated, feature is to be imaged in photoresist the process described above may be used. To achieve selective removal of the assist features, it is necessary that the isolated feature be attached in some way to photoresist that has not been completely undercut. This could be accomplished as described above or the assist features could be made narrower than the isolated feature. In cases where this would not be a problem, the isolated feature may have one or two small portions of its length widened, e.g. giving it a barbell shape or it could be briefly widened at only a single, though centrally located, point etc. etc.  
         [0033]    The invention is most useful for the imaging of isolated features when, prior to adding the assist features, there is a pattern density of up to about 40% within a distance of 10 microns from the isolated feature and when, after adding the assist features, the pattern density has been increased to between about 20 and 80%. Application of the invention can be effected without in any way interfering with the application of other aids to improved resolution such as phase shifting masking (particularly for ultra-small resolution requirement, e.g., 90 nm using a 248 nm wavelength light source) or (as noted earlier) off-axis illumination.  
         [0034]    The invention is compatible with current MR head manufacturing processes and no special masks are needed. The design of the assist features can be optimized according to the available illumination tool, making the cost of the mask much less than if OPC had been used. The number of masks needed is also less compared to that of conventional isolated-line design in association with PSM techniques. The process window of the GPC layer is thus increased significantly.