Abstract:
An approach is presented for designing a polymeric layer for nanometer scale thermo-mechanical storage devices. Cross-linked polyimide oligomers are used as the recording layers in atomic force data storage device, giving significantly improved performance when compared to previously reported cross-linked and linear polymers. The cross-linking of the polyimide oligomers may be tuned to match thermal and force parameters required in read-write-erase cycles. Additionally, the cross-linked polyimide oligomers are suitable for use in nano-scale imaging.

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 11/626,431 filed on Jan. 24, 2007 now abandoned which is continuation-in-part of copending U.S. patent application Ser. No. 11/358,774 filed on Feb. 21, 2006. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of high density data storage and imaging and more specifically to a data storage and image medium, a data storage and imaging system, and a data storage and imaging method. 
     BACKGROUND OF THE INVENTION 
     Current data storage and imaging methodologies operate in the micron regime. In an effort to store ever more information in ever smaller spaces, data storage density has been increasing. In an effort to reduce power consumption and increase the speed of operation of integrated circuits, the lithography used to fabricate integrated circuits is pressed toward smaller dimensions and more dense imaging. As data storage size increases and density increases and integrated circuit densities increase, there is a developing need for data storage and imaging methodologies that operate in the nanometer regime. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a method, comprising: heating a probe to at least 100° C.; pushing the heated probe into a cross-linked resin layer of polyimide oligomers; and removing the probe from the resin layer, resulting in formation of a deformed region in the resin layer. 
     A second aspect of the present invention is the first aspect, wherein the polyimide oligomers have the structure: 
                                
wherein R′ is selected from the group consisting of
 
                                
wherein R″ is selected from the group consisting of
 
                                
wherein n is an integer from about 5 to about 50.
 
     A third aspect of the present invention is the second aspect wherein the layer of polyimide oligomers includes a reactive diluent, the reactive diluent selected from the group consisting of: 
                                
where R 1 , R 2  and R 3  are each independently selected from the group consisting of hydrogen, alkyl groups, aryl groups, cycloalkyl groups, alkoxy groups, aryloxy groups, alkylamino groups, arylamino groups, alkylarylamino groups, arylthio, alkylthio groups and
 
                                
wherein the polyimide oligomers are cross-linked by reactive diluent groups derived from the reactive diluent during the curing.
 
     A fourth aspect of the present invention is the third aspect, wherein a glass transition temperature of the resin layer with the reactive diluent groups is within about 50° C. of a glass transition temperature of an otherwise identical resin layer formed without the reactive diluent groups. 
     A fifth aspect of the present invention is the first aspect wherein the polyimide oligomers have the structure:
 
E-R 2             A 1 -A 2 -A 3 - . . . -A N )- R 1 -R 2 -E;
 
wherein E is

                                
wherein each of A 1 , A 2 , A 3  . . . A N  is independently selected from the group consisting of:
 
                                
wherein R 1  is selected from the group consisting of
 
                                
wherein R 2  is selected from the group consisting of
 
                                
wherein R 3  is
 
                                
wherein N is an integer greater than or equal to 2;
 
wherein at least one of A 1 , A 2 , A 3  . . . A N  is
 
                                
wherein at least one of A 1 , A 2 , A 3  . . . A N  is
 
     
       
                 
         
             
             
         
      
     
     A sixth aspect of the present invention is the first aspect wherein after the curing, the resin layer is cross-linked by the reactive endgroups of the polyimide oligomers. 
     A seventh aspect of the present invention is the first aspect wherein the polyimide oligomers include reactive pendent groups attached along backbones of the polyimide oligomers and after the curing, the resin layer is cross-linked by the reactive pendent groups. 
     A eighth aspect of the present invention is the first aspect, wherein a glass transition temperature of the resin layer is less than about 250° C. 
     A ninth aspect of the present invention is the first aspect, wherein a modulus of the resin layer above a glass transition temperature of the resin layer is between about 1.5 and about 3.0 decades lower than a modulus of the resin layer below the glass transition temperature of the resin layer. 
     A tenth aspect of the present invention is the first aspect, wherein the resin layer is thermally and oxidatively stable to at least 400° C. 
     An eleventh aspect of the present invention is the first aspect, further including: removing the resin layer in the deformed region to form an exposed region of a substrate and a region of substrate protected by the resin layer; and modifying at least a portion of the exposed region of substrate. 
     A twelfth aspect of the present invention is the first aspect, further including: dragging the probe in a direction parallel to a top surface of the resin layer while heating and pushing the probe, resulting in formation of a trough in the resin layer. 
     A thirteenth aspect of the present invention is the first aspect, wherein the cross-linked resin layer has a thickness between about 10 nm and about 500 nm and a thickness variation of less than about 1.0 nm across the cross-linked resin layer. 
     A fourteenth aspect of the present invention is a method, comprising: heating a probe to at least 100° C.; pushing the heated probe into a cross-linked resin layer of polyimide oligomers; and removing the probe from the resin layer, resulting in formation of a deformed region in the resin layer. 
     A fifteenth aspect of the present invention is a data storage device, comprising: a recording medium comprising a resin layer overlying a substrate, in which topographical states of the resin layer represent data, the resin layer comprising cross-linked polyimide oligomers; and a read-write head having one or more thermo-mechanical probes, each of the thermo-mechanical probes including a resistive region locally heating a tip of the thermo-mechanical probe in response to electrical current being applied to the thermo-mechanical probe; and a scanning system for scanning the read-write head across a surface of the recording medium. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
         FIGS. 1A through 1C  illustrate the structure and operation of a tip assembly for a data storage device including the data storage medium according to the embodiments of the present invention; 
         FIG. 2  is an isometric view of a local probe storage array including data storage medium according to the embodiments of the present invention; 
         FIGS. 3A through 3D  are cross-section views illustrating formation of a pattern in a substrate according to one embodiment of the present invention; 
         FIGS. 4A through 4E  are cross-section views illustrating formation of a pattern in a substrate according to another embodiment of the present invention; 
         FIGS. 5A through 5E  are cross-section views illustrating formation of a pattern in a layer on a substrate according to an embodiment of the present invention; 
         FIG. 6  is a diagram illustrating cross-linking of a polyimide resin with a reactive diluent according to embodiments of the present invention; 
         FIG. 7  is thermo-gravimetric analysis plotting percentage of weight remaining and temperature versus time of a polyimide resin according to an embodiment of the present invention compared to polystyrene resins; 
         FIG. 8  is a plot of modulus versus temperature of polyimide resins according to embodiments of the present invention; 
         FIGS. 9A through 9D  are SEM photomicrographs of tips of various tip assemblies; and 
         FIG. 10  is an AFM scan-line cross-section showing data bits written in a storage medium according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For the purpose of describing the present invention, on a scale of 0-100 units, a decade is 10 units. On a scale of 0-1000, a decade is 100 units. Therefore a decade of a range is defined as one-tenth of the range of units from 0 units to 10 n  units, wherein n is a whole positive integer equal to or greater than 0. 
       FIGS. 1A through 1C  illustrate the structure and operation of a tip assembly  100  for a data storage device including the data storage medium according to the embodiments of the present invention. In  FIG. 1A , probe tip assembly  100  includes a U-shaped cantilever  105  having flexible members  105 A and  105 B connected to a support structure  110 . Flexing of members  105 A and  105 B provides for substantial pivotal motion of cantilever  105  about a pivot axis  115 . Cantilever  105  includes a tip  120  fixed to a heater  125  connected between flexing members  105 A and  105 B. Flexing members  105 A and  105 B and heater  125  are electrically conductive and connected to wires (not shown) in support structure  110 . In one example, flexing members  105 A and  105 B and tip  120  are formed of highly-doped silicon and have a low electrical resistance, and heater  125  is formed of lightly doped silicon having a high electrical resistance sufficient to heat tip  120 , in one example, between about 100° C. and about 400° C. when current is passed through heater  125 . The electrical resistance of heater  125  is a function of temperature. 
     Also illustrated in  FIG. 1A  is a storage medium (or a recording medium)  130  comprising a substrate  130 A, and a cured polyimide resin layer  130 B. In one example, substrate  130 A comprises silicon. Cured polyimide resin layer  130 B may be formed by solution coating, spin coating, dip coating or meniscus coating uncured polyimide resin formulations and performing a curing operation on the resultant coating. In one example, cured polyimide resin layer  130 B has a thickness between about 10 nm and about 500 nm and a variation in thickness of less than about 1.0 nm across the cured polyimide resin layer. The composition of cured polyimide resin layer  130 B is described infra. An optional penetration stop layer  130 C is shown between cured polyimide resin layer  130 B and substrate  130 A. Penetration stop layer  130 C limits the depth of penetration of tip  120  into cured polyimide resin layer  130 B. 
     Turning to the operation of tip assembly  100 , in  FIG. 1A , an indentation  135  is formed in cured polyimide resin layer  130 B by heating tip  120  to a writing temperature T W  by passing a current through cantilever  105  and pressing tip  120  into cured polyimide resin layer  130 B. Heating tip  120  allows the tip to penetrate the cured polyimide resin layer  130 B forming indentation  135 , which remains after the tip is removed. In one example, the cured polyimide resin layer  130 B is heated to above 200° C. by heated tip  120  to form indentation  135 . As indentations  135  are formed, a ring  135 A of cured polyimide polymer is formed around the indentation. Indentation  135  represents a data bit value of “1”, a data bit value of “0” being represented by an absence of an indentation. 
       FIGS. 1B and 1C  illustrate reading the bit value. In  FIGS. 1B and 1C , tip assembly  100  is scanned across a portion of cured polyimide resin layer  130 B. When tip  120  is over a region of cured polyimide resin layer  130 B not containing an indentation, heater  125  is a distance D 1  from the surface of the cured polyimide resin layer (see  FIG. 1B ). When tip  120  is over a region of cured polyimide resin layer  130 B containing an indentation, heater  125  is a distance D 2  from the surface of the cured polyimide resin layer (see  FIG. 1C ) because the tip “falls” into the indentation. D 1  is greater than D 2 . If heater  125  is at a temperature T R  (read temperature), which is lower than T W  (write temperature), there is more heat loss to substrate  130 A when tip  120  is in an indentation than when the tip is not. This can be measured as a change in resistance of the heater at constant current, thus “reading” the data bit value. It is advantageous to use a separate heater for reading which is mechanically coupled to the tip but thermally isolated from the tip. A typical embodiment is disclosed in Patent Application EP 05405018.2, 13 Jan. 2005. 
     “Erasing” (not shown) is accomplished by positioning tip  120  in close proximity to indentation  135 , heating the tip to a temperature T E  (erase temperature), and applying a loading force similar to writing, which causes the previously written indent to relax to a flat state whereas a new indent is written slightly displaced with respect to the erased indent. The cycle is repeated as needed for erasing a stream of bits whereby an indent always remains at the end of the erase track. T E  is typically greater than T W . The erase pitch is typically on the order of the rim radius. In one example, the cured polyimide resin layer  130 B is heated to above about 200° C. by heated tip  120 , and the erase pitch is 10 nm to eliminate indentation  135 . 
       FIG. 2  is an isometric view of a local probe storage array  140  including data storage medium according to the embodiments of the present invention. In  FIG. 2 , local probe storage array  140  includes substrate  145  having a cured polyimide resin layer  150  (similar to cured polyimide resin layer  130 B of  FIGS. 1A ,  1 B and  1 C), which acts as the data-recording layer. An optional tip penetration stop layer may be formed between cured polyimide resin layer  150  and substrate  145 . In one example, substrate  145  comprises silicon. Cured polyimide resin layer  150  may be formed by solution coating, spin coating, dip coating or meniscus coating uncured polyimide resin formulations and performing a curing operation on the resultant coating. In one example, cured polyimide resin layer  150  has a thickness between about 10 nm and about 500 nm and a variation in thickness across a writeable region of cured polyimide resin layer  150  of less than about 1.0 nm across the cured polyimide resin layer. The composition of cured polyimide resin layer  150  is described infra. Positioned over cured polyimide resin layer  150  is a probe assembly  155  including an array of probe tip assemblies  100  (described supra). Probe assembly  155  may be moved in the X, Y and Z directions relative to substrate  145  and cured polyimide resin layer  150  by any number of devices as is known in the art. Switching arrays  160 A and  160 B are connected to respective rows (X-direction) and columns (Y-direction) of probe tip assemblies  100  in order to allow addressing of individual probe tip assemblies. Switching arrays  160 A and  160 B are connected to a controller  165  which includes a write control circuit for independently writing data bits with each probe tip assembly  100 , a read control circuit for independently reading data bits with each probe tip assembly  100 , an erase control circuit for independently erasing data bits with each probe tip assembly  100 , a heat control circuit for independently controlling each heater of each of the probe tip assembles  100 , and X, Y and Z control circuits for controlling the X, Y and Z movement of probe assembly  155 . The Z control circuit controls a contact mechanism (not shown) for contacting the cured polyimide resin layer  150  with the tips of the array of probe tip assemblies  100 . 
     During a write operation, probe assembly  155  is brought into proximity to cured polyimide resin layer  150  and probe tip assemblies  100  are scanned relative to the cured polyimide resin layer. Local indentations  135  are formed as described supra. Each of the probe tip assemblies  100  writes only in a corresponding region  170  of cured polyimide resin layer  150 . This reduces the amount of travel and thus time required for writing data. 
     During a read operation, probe assembly  155  is brought into proximity to cured polyimide resin layer  150  and probe tip assemblies  100  are scanned relative to the cured polyimide resin layer. Local indentations  135  are detected as described supra. Each of the probe tip assemblies  100  reads only in a corresponding region  170  of cured polyimide resin layer  150 . This reduces the amount of travel and thus the time required for reading data. 
     During an erase operation, probe assembly  155  is brought into proximity to cured polyimide resin layer  150 , and probe tip assemblies  100  are scanned relative to the cured polyimide resin layer. Local indentations  135  are erased as described supra. Each of the probe tip assemblies  100  reads only in a corresponding region  170  of cured polyimide resin layer  150 . This reduces the amount of travel and thus time required for erasing data. 
     Additional details relating to data storage devices described supra may be found in the articles “ The Millipede—More than one thousand tips for future AFM data storage ,” P. Vettiger et al.,  IBM Journal of Research and Development . Vol. 44 No. 3, May 2000 and “ The Millipede—Nanotechnology Entering Data Storage ,” P. Vettiger et al.,  IEEE Transaction on Nanotechnology , Vol. 1, No, 1, March 2002. See also United States Patent Publication 2005/0047307, Published Mar. 3, 2005 to Frommer et al. and United States Patent Publication 2005/0050258, Published Mar. 3, 2005 to Frommer et al., both of which are hereby included by reference in there entireties. 
       FIGS. 3A through 3D  are cross-section views illustrating formation of a pattern in a substrate according to one embodiment of the present invention. In  FIG. 3A , formed on a substrate  200  is a cured polyimide resin layer  205  (similar to cured polyimide resin layer  130 B of  FIGS. 1A ,  1 B and  1 C and cured polyimide resin layer  150  of  FIG. 2 ) which will be an imaging layer. Cured polyimide resin layer  205  may be formed by applying (by solution coating, spin coating, dip coating or meniscus coating) a layer of uncured polyimide oligomers (including reactive end capping agents and optional reactive diluents or reactive backbone linking agents as described infra) and then heating the substrate and uncured polyimide oligomers to a curing temperature causing cross-linking of the polyimide oligomers into a polyimide resin. 
     In  FIG. 3B , a heated probe tip  210  is pushed down (perpendicular to a top surface  215  of substrate  200 ) into cured polyimide resin layer  205  and then dragged parallel to top surface  215  of substrate  200  thus exposing a region of substrate  200 . 
     In  FIG. 3C , a trench  220  is etched into substrate  200  wherever the substrate is not protected by cured polyimide resin layer  205 . In  FIG. 3D , cured polyimide resin layer  205  (see  FIG. 3C ) is removed. 
       FIGS. 4A through 4E  are cross-section views illustrating formation of a pattern in a substrate according to another embodiment of the present invention.  FIGS. 4A  are  4 B are similar to  FIGS. 3A and 3B  except in  FIG. 4B , heated probe  210  is not pressed completely through cured polyimide resin layer  205  forming a cured polyimide resin thinned region  225  in cured polyimide resin layer  205 . In  FIG. 4C , cured polyimide resin thinned region  225  (see  FIG. 4B ) is removed exposing top surface  215  of substrate  200  and also producing a thinned cured polyimide resin layer  205 A. In one example, the removal of cured polyimide resin thinned region  225  is done by reactive plasma. In one example, the removal of cured polyimide resin thinned region  225  is done by controlled exposure to a liquid or a vapor. 
     In  FIG. 4D , trench  220  is etched into substrate  200  wherever the substrate is not protected by thinned cured polyimide resin layer  205 A. In  FIG. 4E , thinned cured polyimide resin layer  205 A (see  FIG. 4D ) is removed. 
       FIGS. 5A through 5E  are cross-section views illustrating formation of a pattern in a layer on a substrate according to an embodiment of the present invention.  FIGS. 5A and 5B  are similar to  FIGS. 4A and 4B  except a hard mask layer  230  is formed between substrate  200  and cured polyimide resin layer  205 . In  FIG. 5C , cured polyimide resin thinned region  225  (see  FIG. 5B ) is removed exposing a top surface  235  of hard mask layer  225  and also producing a thinned cured polyimide resin layer  205 A. 
     In  FIG. 5D , trench  240  is etched into hardmask layer  225  wherever the substrate is not protected by thinned cured polyimide resin layer  205 A. In  FIG. 5E , thinned cured polyimide resin layer  205 A (see  FIG. 5D ) is removed. Hardmask layer  230  may be used to etch substrate  200  or to block diffusion and ion implantation or as a mandrel for deposition of other coatings including conformal coatings. 
     The methodologies illustrated in  FIGS. 3A through 3D ,  4 A through  4 E and  5 A through  5 E may advantageously be applied to fabrication of integrated circuits and other semiconductor devices. Using these methods, features having a minimum dimension of less than about 40 nm may be formed. 
     Turning to the composition of cured polyimide resin layer  130 B of  FIGS. 1A through 1C , cured polyimide resin layer  150  of  FIG. 2  and cured polyimide resin layer  205  of  FIGS. 3A through 3C ,  FIGS. 4A and 4B  and  FIGS. 5A and 5B , there are three general formulations of uncured polyimide resins. It should be understood that for the purposes of the present invention curing an oligomer implies cross-linking the oligomer to form a polymer or cross-linked polymer or resin. 
     The polyimide medium or imaging layer of the embodiments of the present invention advantageously meets certain criteria. These criteria include high thermal stability to withstand millions of write and erase events, low wear properties (low pickup of material by tips), low abrasion (tips do not wear out easily), low viscosity for writing, glassy character with little or no secondary relaxations for long data bit lifetime, and shape memory for erasability. 
     Thermal and oxidative stability was imparted to cured polyimide resins by incorporating a large aromatic content in the polymer backbone and by ladder type linkages such as imide functionalities. Cured polyimide resins according to embodiments of the present invention have high temperature stability while maintaining a low glass transition temperature (T g ), which is contrary to current teaching that high temperature stability results in a high T g  and vice versa. In one example, cured polyimide resins according to embodiments of the present invention are thermally and oxidatively stable to at least 400° C. 
     Wear and erasability of the media were improved by cross-linking the polyimide oligomers without increasing the T g  which was unexpected. By placing the cross-linking sites at the chain ends, the molecular weight of polyimide oligomers is predefined and therefore cross-linking was found to have a lesser effect upon the glass transition temperature than is currently thought. The width of the transition from the rubbery to glassy state of the cured polyimide resin was found not to increase significantly over that of the polyimide oligomer. The sharp and practically temperature-invariant transition from the glassy to rubbery state as seen in polyimide oligomers was maintained in the cross-linked resin. Again, this is contrary to what is currently thought. The molecular weights of the polyimide oligomers themselves are controlled by the ratio of anhydride, amine and reactive endgroup precursor used in the polyimide oligomer synthesis. 
     Further control over the cross-link density was achieved by adding controlled amounts of reactant diluents described infra that enhance cross-linking. These reactive diluents formed a high density of cross-links that enhanced the wear properties of the polyimide medium without greatly increasing the T g  or width of the glass transition. 
     The glass transition temperature was adjusted for good write performance. To optimize the efficiency of the write process there should be a sharp transition from the glassy state to the rubbery state. A sharp transition allows the cured resin to flow easily when a hot tip is brought into contact and quickly return to the glassy state once the hot tip is removed. However, too high a T g  leads to high write currents and damage to the probe tip assemblies described supra. Incorporation of flexible aryl ether and thioether linkages resulted in polyimide resins of lower than expected T g . In one example, cured polyimide resins of the embodiments of the present invention have T g s of less than about 250° C., preferably between about 120° C. and about 250° C., more preferably between about 120° C. and 150° C. 
     Long data bit lifetime of the polyimide resin medium was obtained by the incorporation of hetero-atoms such as oxygen and sulfur in the polyimide resin backbone and varying the catenation of aromatic rings from para to meta linkages. 
     A first formulation of uncured polyimide resin comprises polyimide oligomers having the structure: 
                                
wherein R′ is selected from the group consisting of
 
                                
wherein R″ is selected from the group consisting of
 
                                
wherein n is an integer from about 5 to about 50.
 
     The endgroups, having the structure: 
                                
provide the cross-linking of the polyimide oligomers into a polyimide resin. The reactive endgroup is the phenylethynyl group of structure (X 1 ). In one example, curing is performed at about 300° C. to about 350° C. In one example, polyimide oligomers according to the first formulation (and of the second formulation described infra) advantageously have a molecular weight of from about 4,000 Daltons to about 15,000 Daltons.
 
     In a second formulation of uncured polyimide resin, one or more of the following reactive diluents (including combinations of different structures (XII)) is added to the first formulation: 
                                
where R 1 , R 2  and R 3  are each independently selected from the group consisting of hydrogen, alkyl groups, aryl groups, cycloalkyl groups, alkoxy groups, aryloxy groups, alkylamino groups, arylamino groups, alkylarylamino groups, arylthio, alkylthio groups and
 
     
       
                 
         
             
             
         
      
     
     It should be noted that reactive diluents XII and XIII contain three substituted phenylethynyl groups. The phenylethynyl groups of the polyimide oligomers and the phenylethynyl groups reactive diluents provide the cross-linking of the polyimide oligomers into a polyimide resin. In one example, curing is performed at about 300° C. to about 350° C. 
     In one example, a Tg of a cured polyimide resin layer formed using the second formulation of the present invention with a reactive diluent is within about 50° C. of a Tg of an otherwise identical cured polyimide resin layer formed without the reactive diluent. 
       FIG. 6  is a diagram illustrating cross-linking of a polyimide resin with a reactive diluent according to embodiments of the present invention. In  FIG. 6 , a mixture of straight chain polyimide oligomer  250  of repeating units n and having two reactive endgroups  255  (which represents structure (I)) and a reactive diluent  260  having three reactive functionalities  265  (representing structures (XII and XIII) is heat cured to produce a cross-linked polyimide resin  270 . In resin  270 , polyimide oligomers  250  are linked to each other through respective reactive endgroups; polyimide oligomers  250  are linked to reactive diluents  260  through respective reactive endgroups and reactive diluents  260  and linked to each other through respective reactive endgroups. Although Tg is usually a function of molecular weight and cross-link density, in this case it is largely independent of the percentage by weight of reactive diluent in the polyimide oligomer/reactive diluent mixture. 
     A third formulation of uncured polyimide resin comprises linear polyimide oligomers having the structure:
 
E-R 2             A 1 -A 2 -A 3 - . . . -A N )-R 1 -R 2 -E;  (XIV)
 
wherein E is

                                
wherein each of A 1 , A 2 , A 3  . . . A N  is independently selected from the group consisting of:
 
                                
wherein R 1  is selected from the group consisting of
 
                                
wherein R 2  is selected from the group consisting of
 
                                
wherein N is an integer greater than or equal to 2;
 
wherein at least one of A 1 , A 2 , A 3  . . . A N  is
 
                                
wherein at least one of A 1 , A 2 , A 3  . . . A N  is
 
     
       
                 
         
             
             
         
      
     
     In one example, polyimide oligomers according to the third formulation advantageously have a molecular weight of from about 4,000 Daltons to about 15,000 Daltons. In one example the integer value of N in structure (XIV) is consistent with a molecular weight of from about 4,000 Daltons to about 15,000 Daltons. In one example, the value of N in structure (XIV) is no greater than about 50. 
     It should be understood that in structure (XIV) the monomers represented by structures (XVI) and (XVII)) may be arranged in a linear sequence with (a) all structures (XVI) in one subsequence and all structures (XIV) in another subsequence, (b) in an alternating sequence, (c) in other regular repeating sequences or (d) in random sequence. 
     EXPERIMENTAL 
     All materials were purchased from Aldrich and used without further purification unless otherwise noted. 
     Preparation of Thioether Dianhydride Oligomers (mTEDA and pTEDA) 
     Either 1,3-benzenedithiol or 1,4-benzenedithiol was dissolved in DMSO (20% solids) with triethylamine and 4-fluorophthalic anhydride. The mixture was heated to 60° C. for 4 hours and then either the mTEDA or pTEDA was precipitated on ice, filtered, and re-crystallized twice from DMSO/acetic anhydride. 
     Preparation of Phenylene Ether Dianhydride Oligomers 
     A bisphenol (e.g. 4-hydroxyphenyl ether) was dissolved in dry DMF with 4-nitrophthalonitrile and potassium carbonate. The solution was heated to 120° C. and the water generated was removed by azeotropic distillation with toluene. After 24 hours, the solids were precipitated on ice. The resulting solid was collected by vacuum filtration. The solid was then refluxed in toluene, ethanol, and hydrochloric acid to hydrolyze the nitrile groups to carboxylic acids. The mixture was again poured over ice and the resulting solid collected by vacuum filtration. The tetraacid was then dissolved in toluene and acetic anhydride, and heated to reflux for 8 hours. The resulting precipitate was collected by vacuum filtration and re-crystallized from acetic anhydride. 
     Preparation of Diamines 
     A bisphenol was dissolved in dry DMF with 4-fluoronitrobenzene, and potassium carbonate. The same procedure was followed as above for the nucleophilic aromatic substitution. The resulting solid was dissolved in THF, and NaBH 4  was added slowly. The reaction was allowed to stir overnight and the product was collected by removal of the solvent under vacuum, and then extracted with CH 2 Cl 2  and water. The organic phase was collected and the solvent removed under vacuum. The resulting solid was purified by vacuum sublimation. 
     Preparation of Bishydroxyphenylethers 
     The reagent 3-bromophenol was reacted with benzylbromide in the presence of potassium carbonate and 18-crown-6 in THF for 24 h. The reaction mixture was filtered to remove excess potassium carbonate and resultant potassium bromide, and the solvent was removed under vacuum. The remaining liquid was filtered through silica to give 3-bromophenylbenzylether in 92% yield. This product was then dissolved in dry NMP together with resorcinol, copper iodide, cesium carbonate, and tetramethylheptanedione. The mixture was stirred vigorously and heated at 120° C. for 72 hours. The solution was then precipitated by pouring over ice and extracted with methylene chloride. The organic phase was collected and the solvent removed. The resulting oil was dissolved in toluene and concentrated hydrochloric acid and heated to reflux. 
     Polyimide Oligomer Synthesis from Amic Acid 
     In a dry atmosphere, the oligomers, a diamine, and acetic anhydride were dissolved in dry cyclohexanone (20% solids) and allowed to stir for 24 hours. The poly(amic acid) formed was used to cast films from cyclohexanone. NMR spectra of the amic acids were acquired by removal of the solvent under vacuum and the addition of dry DMSO-d 8 . 
     Polymide Oligomer Synthesis by Chemical Imidization 
     Under an inert atmosphere, a bisanhydride and a diamine (purified by vacuum sublimation) were dissolved in dry NMP and allowed to stir for 24 hours. Acetic anhydride and triethylamine were then added and the reaction was allowed to stir under inert atmosphere for 48 hours. Finally the mixture was heated to 60° C. for 2 hours and then precipitated by pouring into stirring methanol. The resulting solid was washed on the frit with water, and methanol, and re-precipitated twice from cyclohexanone (or NMP). 
     Film Preparation from Polyimide 
     The polymer was dissolved in cyclohexanone (5% by weight) and filtered through a 0.2-micrometer filter onto UV/ozone cleaned silicone wafers. The wafer was then spun at 2500 rpm for 30 seconds yielding an approximately 100 nm thick film. The films were cross-linked on a hotplate under an inert atmosphere with a heating program of a 1-hour ramp from 50° C. to 350° C. and held an additional hour at 350° C. Bulk films and samples containing reactive diluent structure (XIII) were prepared in a similar fashion except for bulk films where a 20 weight % solution was used. 
     Film Preparation from Amic Acid 
     Under dry atmosphere, the polyamic acid precursors were diluted with cyclohexanone to the appropriate concentration (5% solids). Minimizing the exposure to ambient air, films of the precursor were spun at 2500 rpm for 30 seconds and then cured with a heating program of a 1-hour ramp from 50° C. to 350° C. and held an additional hour at 350° C. 
     In a first synthesis example, polyimide resins of varying molecular weights were synthesized by varying the ratios of the two oligomers 1,3-bis(4-aminophenoxy)benzene (XXVIII) and 4,4′(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) (XXIX) and the end capping agent 4-phenylethynylpthalic anhydride (XXX). Bulk samples and thin films of these materials were prepared and then cured at 350° C. for one hour to yield highly cross-linked films. 
     
       
                 
         
             
             
         
      
     
     One preparation (Sample A) of structure (XXXI) was extensively studied. In structure (XXXI), the integer value of n is consistent with a molecular weight of about 14,400 Daltons. Cured sample A had a &lt;Mn&gt; 14,400 g/mol, Mw/Mn=1.9 and when cured at 350° C. had a Tg of about 175° C. In structure (XXXI), the integer value of n is consistent with a molecular weight of about 14,400 Daltons. 
     In a second synthesis example: 
     
       
                 
         
             
             
         
      
       
                 
         
             
             
         
       
     
     It should be understood that in structure (XXXIII) the first monomer which is enclosed by ( ) n  and the second monomer which is enclosed by ( ) m  should not be construed as limiting structure (XXXIII) to any particular sequence of first and second monomers. Structure (XXXIII) includes structures where the first and second monomers may be arranged in a linear sequence (a) with all first monomers in one subsequence and all second monomers in another subsequence, (b) in an alternating sequence, (c) in other regular repeating sequences or (d) in random sequence. 
     Other Syntheses 
     In order to reduce the glass transition temperature, the rigidity of the polymer backbone must be decreased. To that end, polyimide oligomers with an increased number of flexible aryl ether linkages as well as thioether linkages were synthesized. 
     Dianhydride phenylene ether containing oligomers was synthesized from the reaction of 4-nitrophthalonitrile with the requisite bisphenol precursor followed by hydrolysis of the cyano groups and dehydration to form the cyclic anhydride. The thioether variants were synthesized directly in one step from the reaction of a bisthiophenol with 4-fluorophthalic anhydride. This synthetic scheme allowed a series of ether- and thioether-containing oligomers with two or more ether or thioether linkages and all possible combinations of meta and para catenation. Furthermore the scheme was easily adapted to synthesize a number of phenylene ethers containing diamines with the same variation on number of ether linkages and catenation schemes by reaction of a bisphenol derivative with 4-fluoronitrobenzene and subsequent reduction of the nitro group to an amine. 
     The thioether dianhydrides were reacted with a series of diamines and 4-phenylethynylphthalic anhydride in specific ratios to yield polyimide oligomers with molecular weights ranging from 4×10 3  g/mol to 10×10 3  g/mol. The first step in the polymerization mechanism is the reaction of one diamine with one anhydride to form an amic acid. One of two steps can be taken at this point. For polymers where the fully imidized form exhibited good solubility and good film forming properties with cyclohexanone as the solvent, the polymer was imidized by a chemical dehydration with triethyl amine and acetic anhydride, and then isolated and characterized. With certain polymer compositions, the fully imidized material was difficult to process. To circumvent these issues with solubility and film forming properties, these polymers were processed into thin films from the amic acid. The polymers were then imidized thermally as thin films concurrently with the final cross-linking reaction. The amic acid precursors were analyzed by removal of the solvent under vacuum and transferred to dry sample containers with dried and distilled solvents for analysis by GPC and  1 H-NMR. The thermal and mechanical properties of cured films were studied by TGA, DSC, and DMA. 
     Example of synthesis and structures of thioether containing dianhydride oligomers: 
                                                                       TABLE I                   Properties of thioether based polyimides                                                              &lt;M n &gt; ×   T g  before           Dianhydride   Diamine   10 −3  g/mol   cure ° C.   T g  cured ° C.                    pTEDA   APTE   4.0   T m  261 (a)   162       mTEDA   APTE   4.0   163   178       mTEDA   mAPB   7.0   (b)   (b)       mTEDA   mAPB   7.0   (c)   209                       Processed from                       amic acid       mTEDA   mAPB   14.0   (c)   151                       Processed from                       amic acid                    where                                                                           (a) T m  indicates the temperature at which the sample melted.       (b) Semicrystalline, Tg not available.       (c) In processing from amic acid, cross-linking occurs concurrently       with the conversion of the acid to the polyimide. Therefore there is       no opportunity to measure Tg previous to cross-linking.            
Example synthesis of bis-4,4′-isophthaloyloxyphenylene ether.
 
     
       
                 
         
             
             
         
      
     
     The phenylene ether materials exhibited similar solubility limitations as the thioether based materials. However, when all linkages in the diamine and the dianhydride were meta catenated, materials showed much improved solubility in solvents such as THF and cyclohexanone. These materials could be processed either from the amic acid or fully imidized states. Exclusively para catenated materials also exhibited semi-crystalline properties. However, once cured, the films were no longer crystalline due to the cross-links preventing crystallization of the chains. Working from the amic acid precursors avoided all solubility issues associated with the para-arylene ether polymers. 
                                                                 TABLE 2                   Phenylene ether based polyimides, imidized chemically            % II   % III   &lt;M n &gt; × 10 −3 g/mol   T g  before cure ° C.   T g  cured ° C.                    100   0   10.0   T m 261   162       50   50   10.0   163   178                    
where
 
                                                                                      R′   R″                                                                                                                                      
and the integer value of n is consistent with a molecular weight of about 10,000 Daltons.
 
       FIG. 7  is thermo-gravimetric analysis(TGA) plotting percentage of weight remaining and temperature versus time of a polyimide resin according to an embodiment of the present invention compared to polystyrene resins. The primary limiting factor in the use of polystyrene (PS) or of polystyrene-co-vinylbenzocyclobutene (PSBCB) for a storage medium was poor thermal stability. The results of a TGA study showed that polyimides resins outperformed PS and PSBCB resins. The styrenic material began to decompose rapidly once the furnace reached 250° C. while the polyimide resin showed no appreciable degradation until above 350° C. in scanning TGA and no weight loss over hours at 250° C. in isothermal TGA. The polyimide resin used in this TGA study was (XXXI). 
       FIG. 8  is a plot of modulus versus temperature polyimide resins according to embodiments of the present invention.  FIG. 8  plots the storage modulus versus temperature for cured sample A and for a polyimide resin made by curing sample A with 30% by weight reactive diluent structure (XIII). Cured sample A exhibited a change in modulus of about 3 decades transitioning from the glassy state to the rubbery state. Cured sample A and 30% by weight reactive diluent structure (XIII) exhibited a drop in modulus of about 2 decades transitioning from the glassy state to the rubbery state. The Tg range for both samples was about the same with a Tg of about 175° C. In general a polyimide resin layer according to embodiments of the present invention had a modulus above a glass transition temperature between about 1.5 and about 3.0 decades lower than a modulus of the polyimide resin layer below the glass transition temperature of the polyimide resin layer. 
       FIGS. 9A through 9D  are SEM photomicrographs of tips of various tip assemblies.  FIG. 9A  is an SEM photomicrograph of an unused tip  120  (see  FIG. 1A ).  FIG. 9B  is an SEM photomicrograph of a worn tip  120  after use on a polystyrene layer. 
       FIG. 9C  is an SEM photomicrograph of an worn tip  120  after use on a PSBCB layer showing pickup of the storage medium.  FIG. 9D  is an SEM photomicrograph of tip  120  after about 2.4E6 write/erase and about 2.3E8 read cycles of a polyimide resin medium according to embodiments of the present invention. As can be seen there is virtually no tip wear. 
       FIG. 10  is an AFM scan-line cross-section showing data bits written in a storage medium according to an embodiment of the present invention. In  FIG. 10  a pattern of data bits (indentation for a “1”, no indentation for a “0”) were written and the definition of the data determined using an AFM. Each “1” bit generated a very sharp and distinct signal, while the noise generated by “0” bits was very low. The write pitch was 34 nm which is greater than 500 Gb/inch 2 . 
     Thus, the embodiments of the present invention provide data storage and imaging methodologies that operate in the nanometer regime. 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.