Abstract:
A technique is disclosed that combines a bilayered photoresist structure, similar to that which is already in use in the MR head industry, with a post development UV irradiation treatment which reduces the manufacturable feature-size to be below the resolution limit. The technique is compatible with current manufacturing processes, requires no additional investment, and can be extended to ultra-small feature sizes.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the general field of photolithography with particular reference to post development treatment and CD reduction.  
       BACKGROUND OF THE INVENTION  
       [0002]     With current KrF photolithography, it is difficult to achieve patterns in photoresist in which the critical dimension is below about 125 nm while retaining adequate manufacturing process latitude. In particular, the manufacture of magneto-resistive (MR) heads for high-density storage applications calls for ever decreasing dimensions of the reader/writer sensors which, in many cases, extend beyond the capability of current lithographic equipment, necessitating the incorporation of resolution-enhancement techniques such as phase shifting masks (PSM), optical proximity correction (OPC), and off-axis illumination.  
         [0003]     These techniques, however, greatly increase process complexity and manufacturing cost in exchange for marginal reductions in the manufacturable feature size. Current lithographic equipment is capable of achieving a minimum feature-size of 180 nm while maintaining manufacturable process latitude. With the help of OPC and off-axis illumination the minimum size may reducible to 120 nm. Further reductions below 120 nm will require immense investment in PSM and/or next-generation lithographic tools.  
         [0004]     Eisele et al. (U.S. Pat. No. 6,117,622) teach a technique that utilizes post-development UV irradiation and bake of chemically amplified photoresist patterns to shrink the resist features by up to 25%. Their technique, however, has limited practical value because 
        1. it shrinks the resist unevenly in a direction perpendicular to the substrate; i.e., the top of the resist shrinks to a much greater extent than the bottom, where the adhesion between the resist and substrate prevents effective shrinking;     2. it calls for a high-temperature bake, up to 230° C., after irradiation to flow the resist to achieve large shrinkage, which reduces the uniformity of resist dimensions and is often deleterious to the overall process; and     3. upon UV irradiation and bake, photoresists tend to become insoluble in stripping solutions, requiring additional aggressive resist-removal processes that greatly limit the applicability of their technique.        
 
         [0008]     A routine search of the prior art was performed with the following references of interest being found:  
         [0009]     In U.S. Pat. No. 6,348,301, Lin describes linear reduction of a PR mask using two baking steps. In U.S. Pat. No. 8,340,556, Wong discloses electron beam radiation to reduce line width of a pattern and, in U.S. Pat. No. 6,576,405, Buffat et al. teach UV exposure and post-exposure baking.  
       SUMMARY OF THE INVENTION  
       [0010]     It has been an object of at least one embodiment of the present invention to provide an etch mask whose minimum width is less than the critical dimension of the optical projection system used in its formation.  
         [0011]     Another object of at least one embodiment of the present invention has been to provide a process for manufacturing said etch mask.  
         [0012]     Still another object of at least one embodiment of the present invention has been that said process be compatible with current MR head manufacturing processes.  
         [0013]     A further object of at least one embodiment of the present invention has been that said process be cost effective, flexible, and extendible to future generations of MR heads.  
         [0014]     These objects have been achieved by utilizing post-development UV irradiation of bilayered photo-resist patterns to controllably and uniformly shrink the resist. The technique does not involve high-temperature processing, and allows treated resists to be later removed by regular stripping process. Since bilayer structures are already in widespread use for patterning of MR structures, the invented process is fully compatible with existing manufacturing processes. Additionally, it is flexible, cost effective, and can be further engineered to accommodate future tighter specifications (of etched line width) as the state of the art advances. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIGS. 1 and 2  plot resist line width as a function of UV irradiation energy.  
         [0016]      FIGS. 3 and 4  are schematic cross-sections to illustrate the key steps of the process of the present invention.  
         [0017]      FIGS. 3A and 4A  illustrate the case where the underlayer is completely removed.  
         [0018]      FIGS. 3B and 4B  illustrate deteriorated and undesirable resist profiles.  
         [0019]      FIG. 5  is a plan view of the structure seen in  FIG. 3 .  
         [0020]      FIG. 6  shows a fuller view of the mask structure of which  FIG. 5  is a part. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     The present invention embodies three novel features that, together, overcome the prior art problems discussed earlier: 
        1. Non-thermal densification of chemically amplified photoresist through post development UV irradiation     2. Addition of an under-layer, impervious to UV irradiation, to facilitate later stripping of the resist     3. Selection of an under-layer with rapid dissolution rate in the developer solution whereby cantilevered edges for the resist are created so that it can shrink without constraint and hence not change its shape.        
 
         [0025]     We now elaborate further on these features, as follows: 
    1. Densification of resist: Chemically amplified resists densify upon post-development UV irradiation due to two speculative mechanisms:     i. Thermal densification: UV irradiation activates photo-acid generators (PAG) which, when subjected to a subsequent bake, de-protect the photo-resist resin attached by solution-inhibiting side groups. The removal of these side groups facilitates denser packing of the polymer chains of the resin, resulting in a reduction in the resist volume.     ii. Non-thermal densification: UV irradiation ruptures chemical bonds of the resist components and creates free radicals, which cross-links the polymer chains of the resist, causing additional densification. These two mechanisms illustrated in  FIG. 1  where resist line-width changes with post-development UV irradiation are compared for two cases: (a) bake at 120° C. for 240 seconds after UV irradiation (curve  11 ), and (b) no bake (curve  12 ). When applying UV radiation energy lower than −500 mJ/cm 2 , there was little resist shrinkage in the no-bake case, indicating that the shrinkage mechanism was dominated by thermal densification, as mentioned in (i). As UV irradiation energy was increased above 500 mJ/cm 2 , the difference between (a) and (b) diminished, indicating that a non-thermal mechanism was dominating, as described in (ii).    
 
         [0029]     A change in the shrinkage mechanism at ˜500 mJ/cm 2  was confirmed in a separate experiment where it was found that developed resist irradiated with energy higher than 500 mJ/cm 2  became insoluble in hot NMP stripping solution, evidence of its being cross-linked. The process details are as follows: Resist: I-801 by Shin-Etsu, Thickness: 0.24 microns; Under-layer: LOL-1000 by Shipley, Thickness: 72 nm; UV exposure tool: F300S power supply and LC-6B conveyer by Fusion UV Systems Co.; Wavelength range of UV illumination: 200 to 400 nm. It should be noted that similar shrinkage behavior was also observed with a different chemically amplified photo-resist: UV210 (0.5 microns) by Shipley, as shown in  FIG. 2 , demonstrating that the post development treatment is effective in reducing the resist line width regardless of resist vendor and type.  
         [0000]     2. Addition of the Under-Layer:  
         [0030]     Addition of an under-layer to the photoresist film overcame the two critical drawbacks of the above-mentioned prior art resist-shrinking technique: (1) resist becomes un-strippable, and (2) resist profile changes upon extended UV irradiation.  
         [0031]     Selection criteria for the under-layer include (1) the under-layer must be soluble in stripping solutions, and (2) the under-layer must possess higher solubility than the exposed resist in developer solutions. Although, strictly speaking, the underlayer is not developed in the same sense as the photoresist, it is convenient to refer to it as developing at a faster rate than the photoresist.  
         [0032]     Two materials that were found suitable for the under-layer: were polymethyl-glutarimide (PMGI) (provided by resist manufacturers such as Shipley) and KrF (D01 series resist manufactured by Clariant), but it will be understood that other materials having similar properties could have been used instead, The under-layer was spin-coated onto substrates (typically silicon wafers) followed by resist coating, also by spin coating, although deposition methods other than spin coating could have been used without changing the final result.  
         [0033]      FIG. 3  is a schematic cross-section showing the bilayer structure after development but prior to post development UV irradiation. After underlayer  32  had been deposited onto substrate  31 , photoresist layer  33   a  was laid down and the system exposed using a suitable imaging system in the usual way. Following exposure, the development time was tuned to dissolve away enough of the under-layer to create the cantilevered overhang (typically between about 0.01 and 0.25 microns) seen in the figure. The structure was then exposed to UV radiation  34  in the manner discussed earlier.  
         [0034]     Note that photoresist ( 33   b ), after ultraviolet treatment, does not necessarily shrink to the same dimension as the underlayer  32  although an undercut is still kept in most cases to facilitate the subsequent liftoff process, the resist dimension still being larger than the underlayer. After ultraviolet treatment, the resist acts as the etching mask of the substrate, and the underlayer helps stripping.  
         [0035]     The width of layer  32  can sometimes even reduce to zero, so that the resist actually ends up suspended in air. (see  FIGS. 3A and 4A ) supported by other, much larger, patterns (as in  FIG. 6 ), which do not get fully undercut. When the resist is suspended, additional advantages to the process accrue since, during ultraviolet exposure, the resist can shrink without any constraint, and can thus more easily retain its original rectangular shapes.  
         [0036]     In comparison, applying ultraviolet treatment to the resist without including an underlayer can result in a slanted sidewall to the resist. During ultraviolet exposure, the top of the resist can shrink freely while the bottom of the resist is confined to the interface with the substrate. As shown in  FIGS. 3B and 4B , the resist profile after shrinkage has deteriorated and is undesirable.  
         [0037]      FIG. 4  shows how resist layer  33   a  of  FIG. 3  has shrunk as a result of the post development irradiation to become resist layer  33   b  which is suitable for use as a surface mask.  
         [0038]      FIG. 5  is a plan view of  FIG. 3  showing that the original photoresist line feature had a width of L which could be no smaller than the CD of the pattern (typically about 0.2 microns). After development, under-layer  51  ( 32  in  FIG. 3 ) had been reduced to a width of L′ so that it was overhung by the photoresist by an amount 52 (typically between about 0.01 and 0.25 microns), said overhang being later reduced or eliminated through shrinkage following further UV exposure.  FIG. 6  shows a fuller picture of the photoresist mask of which  FIG. 5  is a part. Here, photoresist layer  51  overlies thin film layer  61 . After etching, a line feature, whose width is less than the CD, will have been formed. A typical structure of this type would be a UV210 resist, about 0.5 microns thick with a 72-nm PMGI underlayer.  
         [0039]     Cantilevering the resist line not only allowed it to shrink uniformly in the vertical direction, but also maintained the line-width uniformity across the wafer. With this bi-layer structure, the average 3-sigma over the mean line-width was maintained at −5% before and after UV irradiation, while it increased to −30% upon irradiation when there was no under-layer. The irradiated resist was lifted off by dissolving the under-layer in regular stripping solutions such as NMP.  
         [0040]     The overall process flow of the resist-shrinking technique presented above features the following advantages: 
    1. Compatibility: Bilayered resist structure are widely used in the manufacturing of MR heads so the present invention is fully compatible with current MR head manufacturing processes. It should be noted, however, that the bilayer structures that are currently in use are for the purpose of forming patterns through liftoff, so the composition, thickness and etch characteristics of the under-layers of the present invention will be different.     2. Cost effectiveness: The present invention requires UV illumination tools, which are already in widespread use by the industry. So no additional equipment is needed.     3. Flexibility: The post-development UV irradiation can be applied to any layers compatible with the bilayered resist structure to reduce feature-size and improve uniformity.     4. Extendability: The post-development UV irradiation is capable of further size reductions of up to 60% when future equipment/process upgrades realize smaller features.