Abstract:
A method for forming a field emission cathode device is disclosed using a peelable photoresist with standard photolithography processes for patterning a deposition mask, except that the peelable photoresist can be peeled away in dry form. The method offers standard photoresist accuracy with the advantage of high patterning resolution for producing carbon nanotube (CNT) field emitter displays. Example methods using a single peelable photoresist layer, and using two distinct layers of photoresist and peelable film, are presented. Since the method does not require wet processes after CNT deposition, it ensures enhanced CNT emitter performance. In addition, an activation process that liberates CNTs can be performed just before a tape lamination and peeling process step. In this manner, all superfluous nanoparticle material remains confined between the tape and photoresist films, which are removed together and properly discarded.

Description:
[0001]     The present application is a continuation-in-part of U.S. patent application Ser. No. 11/124,332, and is also a continuation-in-part of U.S. patent application Ser. No. 10/269,577, which claims priority to provisional patent applications: 60/343,642; 60/348,856; and 60/369,794. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates in general to photolithography, and in particular to applying a peelable photoresist to manufacture carbon nanotube (CNT) cathodes.  
       BACKGROUND INFORMATION  
       [0003]     Carbon nanotube (CNT) cathode structures are highly effective field emitters for generating cathode rays, exhibiting a high emission current at a low threshold voltage. CNT cathodes can be fabricated, using procedures known for manufacturing semiconductors, as a plurality of microcells to generate an array of pixels, which form the basis for a display device, such as a television, or a computer monitor. Fabrication of CNT cathodes into an array of pixels typically requires masks to align the pixels and deposit CNTs in the form of CNT ink onto the pixels.  
         [0004]     There are two well-known methods in the art to deposit the CNT ink, either using a shadow mask to spray or print CNT ink to define pixel areas, or using a standard photolithography process (spin coating; baking; exposing; developing; wet etching; and wet stripping) to define pixels. Each of these two methods has a unique drawback associated with it.  
         [0005]     Using the shadow mask method, the alignment tolerance of the pixels is limited by the mechanical accuracy of positioning the shadow mask. This mask alignment limitation constrains the pixel resolution. Also, there is a gap between the substrate and the shadow mask that causes CNT ink or solution to leak through the mask edge. Referring to  FIG. 1 , as a result, CNT ink  105  can be deposited on the sidewall of the pixel well  110  or between the mask  102  and the device. This contamination by conductive CNT ink  105  in undesirable areas of the device increases the likelihood of short circuits when the contact grid structure for the pixels is subsequently applied during manufacture of the display. The shadow mask method is therefore not suitable for industrial applications involving high volume manufacturing subject to rigorous quality standards.  
         [0006]     Using a standard photolithography process, involving a photoresist coating and wet stripping of the resist, CNT material becomes exposed to a chemical solution and water, which adversely affects the CNT emitter performance. The degradation of the sensitive CNT material caused by wet stripping and wet rinsing results in higher threshold voltages and lower emission currents of the CNT cathode. Therefore, there is a need in the art for a photolithography method which does not rely on wet processes to remove the photoresist mask.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention addresses the foregoing need by providing a method of using a peelable photoresist that can be patterned using photolithography for producing a field emission cathode device. The cathode device is patterned for making matrix-addressable display pixels using carbon nanotube (CNT) ink. The photoresist film can be peeled off after the CNT ink layer is deposited, without exposure of the CNT material to solvents and wet resist stripping steps that normally destroy CNT emission performance as a result of standard photolithography processes.  
         [0008]     The merits of the present invention over the prior art for defining pixel area are manifold. A peelable photoresist provides the more accurate alignment and higher pixel resolution of photolithography as opposed to using shadow masks. A peelable photoresist eliminates the need for a shadow mask and thus eliminates any associated contamination effects of CNT ink becoming deposited in undesirable areas of the device, as can occur using shadow masks. A peelable photoresist eliminates the need for wet processes during photolithography mask removal, and thus preserves high CNT emitter performance of the pixel cathode. Additionally, a peelable photoresist may be used to avoid wet stripping in other applications, such as manufacturing of integrated circuits, where standard photolithography processes are used.  
         [0009]     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0011]      FIG. 1  illustrates deposition of CNT ink using the shadow mask method of the prior art;  
         [0012]      FIGS. 2A and 2B  illustrate the initial two steps of one embodiment of the present invention that implements a single peelable resist layer: coating, exposing, developing the peelable photoresist; and depositing the CNT ink layer;  
         [0013]      FIGS. 2C and 2D  illustrate third and fourth steps of one embodiment of the present invention that implements a single peelable resist layer: activating the CNT ink with nanoparticles; and laminating the tape on top of the existing structure;  
         [0014]      FIGS. 2E and 2F  illustrate fifth and final steps of one embodiment of the present invention that implements a single peelable resist layer: peeling the tape to remove unwanted CNT ink with the photoresist; and the final resulting structure of the CNT ink emitter cathodes;  
         [0015]      FIGS. 3A and 3B  illustrate the first two steps of one embodiment of the present invention that implements peelable resist comprising a peelable layer and a standard photoresist layer: applying the peelable film; and applying the photoresist;  
         [0016]      FIGS. 3C and 3D  illustrate third and fourth steps of one embodiment of the present invention that implements peelable resist comprising a peelable layer and a standard photoresist layer: exposing with UV light; and developing the photoresist;  
         [0017]      FIGS. 3E and 3F  illustrate fifth and sixth steps of one embodiment of the present invention that implements peelable resist comprising a peelable layer and a standard photoresist layer: stripping the photoresist; and depositing the CNT ink layer;  
         [0018]      FIGS. 3G and 3H  illustrate seventh and eighth steps of one embodiment of the present invention that implements peelable resist comprising a peelable layer and a standard photoresist layer: activating the CNT ink with nanoparticles; and laminating the tape on top of the existing structure;  
         [0019]      FIGS. 3F and 3K  illustrate ninth and final steps of one embodiment of the present invention that implements peelable resist comprising a peelable layer and a standard photoresist layer: peeling the tape to remove unwanted CNT ink with the peelable film; and the final resulting structure of the CNT ink emitter cathodes;  
         [0020]      FIG. 4  illustrates a data processing system; and  
         [0021]      FIG. 5  illustrates a portion of a field emission display made using a cathode in a triode configuration.  
     
    
     DETAILED DESCRIPTION  
       [0022]     In the following description, numerous specific details are set forth such as specific substrate materials to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art.  
         [0023]     Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views.  
         [0024]     The present invention provides a method of using a peelable photoresist that can be patterned using photolithography for producing field emission display pixels using CNT ink as the cathode material. The steps of the procedure in one embodiment of the current invention to process a cathode by using peelable photoresist comprising a single photoresist layer are illustrated in  FIGS. 2A through 2F . The steps of the procedure in another embodiment of the current invention to process a cathode by using peelable photoresist comprising a first peelable layer and a second photoactive layer are illustrated in  FIGS. 3A through 3K . Certain nonessential method steps may be omitted or repeated as required in other embodiments.  
         [0025]      FIG. 1  illustrates the result of the prior art method  100  of spraying or printing CNT ink  103 ,  104 ,  105  using a shadow mask  102  to expose only the unmasked portions of the composite structure below the mask  102  for coating with CNT ink. On the substrate  101 , the trace feed line and pixel electrode pad layer  106  is deposited using a conducting paste. Then, the insulator film layer  107  is deposited to isolate between individual pixel cells  110 . A shadow mask  102  is mechanically positioned a distance above the composite structure  101 ,  106 ,  107 . Then, CNT ink  103 ,  104 ,  105  is sprayed or printed over the shadow mask  102 . The problems with the deposition of CNT ink  103 ,  104 ,  105  are illustrated in  FIG. 1 . Ideally, the CNT ink  104 ,  103  is only deposited on the masked  102  and unmasked (on the pixel electrode pads  106 ) portions of the composite structure  101 ,  106 ,  107 . However, it is observed that, due to the distance between the shadow mask  102  and the composite structure  101 ,  106 ,  107 , some CNT ink  105  becomes deposited in inappropriate locations. The contamination effects of the excessive CNT ink  105 , which leaks through the mask edge onto the sidewall of the pixel well  110  or onto the insulating film layer  107 , may include a short circuit in the grid structure for addressing the individual pixels. Also, the mechanical positioning of the shadow mask constrains the pixel resolution that may be attained using this method. For the above-stated reasons, the shadow mask method  100  is rendered unsuitable for industrial scale, high volume manufacturing, where rigorous quality standards are required. In the present invention, a method which overcomes these problems using a peelable photoresist has been developed.  
         [0026]      FIGS. 2A and 2B  illustrate one example method, wherein a single layer peelable photoresist  210  is applied  200 . Referring to  FIG. 2A , on the substrate  101 , the trace feed lines and pixel electrode pad layer  106  is screen printed using a silver conducting paste (DuPont #7713), followed by baking and firing. Then, the insulator film layer  107  is deposited to isolate between individual pixel cells  110  by screen printing an insulating film  107  (DuPont #9370), followed by baking and firing. Next, a peelable photoresist  210  (Transfer Devices xFILM-R) is spin or spray coated on the composite structure  101 ,  106 ,  107 . This is followed by baking, exposing the mask pattern, and developing the photoresist  210 . The result of this process  200  is illustrated in  FIG. 2A . The unmasked portions of the photoresist  210  reveal the centers of the pixel electrode pads  106 . In the next process step  201 , illustrated in  FIG. 2B , a CNT ink is sprayed or printed, resulting in a layer of CNT ink  104  deposited on the photoresist  210 , and a layer of CNT ink  103  deposited on the pixel electrode pads  106  to form the cathode structure. Note that since there is no gap between the photoresist  210  and the CNT ink  104 , no undesired CNT ink  105  is deposited as shown in  FIG. 1 . The next processing step  400  can be the one illustrated by  FIG. 2C , which activates the CNT material  103  by implanting additional nanoparticles  431  (in the current example, CNTs) into the surface  410 ,  411  of the previously deposited CNT ink layer  103 ,  104 . In one example method, the implantation  400  is performed using a micromachining bead-blaster which bombards the surface  410 ,  411  with nanoparticles  431  using a positionable nozzle  440  from a direction  430  normal to the surface. In the bead-blasting method  400 , different implantation scenarios, including various orientations, carrier bead-CNT mixtures, and positioning regimes, may be practiced with the present invention. In this manner, the surface of the CNT emitter  411  is activated due to a higher concentration of nanoparticles  432  embedded into the cathode surface  411 , which enhances cathode performance. Other activation mechanisms may also be possible within the scope of the present invention. Note that a layer  210  in  FIG. 2C  represents the single layer peelable photoresist. As the next process step,  FIG. 2D , illustrates, the lamination  401  of an adhesive tape  420  (3M), comprising a tape layer on one side and an adhesive layer on the other side, is performed on top of the CNT ink  104  deposited on the masked pattern of peelable photoresist  210 . The lamination  401  may be augmented with additional heat or pressure, or a combination thereof, as required in other embodiments. After lamination  401 , the adhesive tape is firmly bonded to the CNT ink layer  104 , which is, in turn, firmly bonded to the masked pattern of peelable photoresist  210 . The last processing step for a single layer photoresist method of the current invention is illustrated in  FIG. 2E ; this step involves peeling the tape from the composite structure below, thereby removing the bonded CNT ink layer  104  along with the peelable photoresist  210 . Note that since the extraneous CNT ink layer  104  is neatly packaged between the adhesive tape  420  and the peelable photoresist  210 , the risk of contaminating the plurality of now finished cathode structures (pixel wells)  110  with CNT ink  104  has been effectively eliminated.  FIG. 2F  illustrates the final product of a single layer photoresist process, a CNT emitter with a plurality of cathode structures, which can be further processed to create a display with addressable pixels.  
         [0027]      FIGS. 3A-3K  illustrate another example method, wherein a peelable resist comprising two layers, a first layer of peelable material  310  and a second layer of photosensitive material  320 , is applied  300 ,  301 . Referring to  FIG. 3A , on the substrate  101 , the trace feed lines and pixel electrode pad layer  106  is screen printed using a silver conducting paste (DuPont #7713), followed by baking and firing. Then, the insulator film layer  107  is deposited to isolate between individual pixel cells  110  by screen printing an insulating film  107  (DuPont #9370), followed by baking and firing. Next, a peelable film layer  310  (Transfer Devices xFILM) is spin or spray coated on the composite structure  101 ,  106 ,  107 , as shown in  FIG. 3A ; immediately thereafter follows spin or spray coating a standard photoresist  320  as shown in  FIG. 3B . Next the composite structure in  FIG. 3B  is baked. Then, as shown in  FIG. 3C , the mask pattern is exposed  302  using UV light  340  and a standard photolithography mask  330 . Next, as shown in  FIG. 3D , the peelable resist layers  310 ,  320  are developed and stripped  303 . Note that this process  303  may utilize standard chemical solutions or wet stripping without degrading the CNT emitter performance, since no CNT ink  103  is present yet. This process  303  exposes the unmasked portions (pixel cells)  110  of the photomask  330 , which reveals the centers of the pixel electrode pads  106 . Thereafter, as shown in  FIG. 3E , the photoresist layer  320  is stripped  304 . Note that this process  304  may utilize standard chemical solutions or wet stripping without degrading the CNT emitter performance, since no CNT ink  103  is present yet. In the subsequent process step  305 , illustrated in  FIG. 3F , a CNT ink is sprayed or printed on the substrate, resulting in a layer of CNT ink  104  deposited on the peelable material  310 , and a layer of CNT ink  103  deposited on the pixel electrode pads  106  to form the cathode structure. Note that since there is no gap between the peelable material  310  and the CNT ink  104 , no undesired CNT ink  105  is deposited as shown in  FIG. 1 . The next processing step  400  can be the one illustrated by  FIG. 3G , which activates the CNT material  103  by implanting additional nanoparticles  431  (in the current example, CNTs) into the surface  410 ,  411  of the previously deposited CNT ink layer  103 ,  104 . In one example method, the implantation  400  is performed using a micromachining bead-blaster which bombards the surface  410 ,  411  with nanoparticles  431  using a positionable nozzle  440  from a direction  430  normal to the surface. In the bead-blasting method  400 , different implantation scenarios, including various orientations, carrier bead-CNT mixtures, and positioning regimes, may be practiced with the present invention. In this manner, the surface of the CNT emitter  411  is activated due to a higher concentration of nanoparticles  432  embedded into the cathode surface  411 , which enhances cathode performance. Other activation mechanisms may also be possible within the scope of the present invention. Note that a layer  310  in  FIG. 3G  represents the peelable film  310 . As the next process step,  FIG. 3H , illustrates, the lamination  401  of an adhesive tape  420  (3M), comprising a tape layer on one side and an adhesive layer on the other side, is performed on top of the CNT ink  104  deposited on the masked pattern of peelable film  310 . The lamination  401  may be augmented with additional heat or pressure, or a combination thereof, as required in other embodiments. After lamination  401 , the adhesive tape is firmly bonded to the CNT ink layer  104 , which is, in turn, firmly bonded to the masked pattern of peelable film  310 . The last processing step in the present example method is illustrated in  FIG. 3J ; this step involves peeling the tape from the composite structure below, thereby removing the bonded CNT ink layer  104  along with the peelable film  310 . Note that since the extraneous CNT ink layer  104  is neatly packaged between the adhesive tape  420  and the peelable film  310 , the risk of contaminating the plurality of now finished cathode structures (pixel wells)  110  with CNT ink  104  has been effectively eliminated.  FIG. 3K  illustrates the final product of the process, a CNT emitter with a plurality of cathode structures, which can be further processed to create a display with addressable pixels.  
         [0028]     Note that the structures in  FIG. 2F  and  FIG. 3K  are identical. Both aforementioned example processes, either using a single layer of peelable photoresist  210 , or using a peelable resist comprising two layers, a first layer of peelable material  310  and a second layer of photosensitive material  320 , may thus be practiced to yield the same final CNT emitter product.  
         [0029]     A representative hardware environment for practicing the present invention is depicted in  FIG. 4 , which illustrates an exemplary hardware configuration of data processing system  513  in accordance with the subject invention having central processing unit (CPU)  510 , such as a conventional microprocessor, and a number of other units interconnected via system bus  512 . Data processing system  513  includes random access memory (RAM)  514 , read only memory (ROM)  516 , and input/output (I/O) adapter  518  for connecting peripheral devices such as disk units  520  and tape drives  540  to bus  512 , user interface adapter  522  for connecting keyboard  524 , mouse  526 , and/or other user interface devices such as a touch screen device (not shown) to bus  512 , communication adapter  534  for connecting data processing system  513  to a data processing network, and display adapter  536  for connecting bus  512  to display device  538 . CPU  510  may include other circuitry not shown herein, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. Display device  538  represents possible embodiments of the present invention.  
         [0030]      FIG. 5  illustrates a portion of a field emission display  538  made using a cathode in a diode configuration, such as created above. Included with the cathode is a conductive layer  106  and the CNT emitter  103 . The anode may be comprised of a glass substrate  612 , and indium tin layer  613 , and a cathodoluminescent layer  614 . An electrical field is set up between the anode and the cathode. Such a display  538  could be utilized within a data processing system  513 , such as illustrated with respect to  FIG. 4 .  
         [0031]     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.