Abstract:
A laser ablation method is utilized to define the channel length of an organic transistor. A substrate is coated with a deposition of a metal or conductive polymer deposition, applied in a thin layer in order to enhance the resolution that can be attained by laser ablation. The laser ablation method can be used in a roll-to-roll process, and achieves speeds, volumes, prices and resolutions that are adequate to produce printed electronic technologies.

Description:
BACKGROUND 
     1. Field of Invention 
     The present invention relates to organic transistors and, more particularly, to a laser ablation method of fabricating high performing organic FETs utilizing an efficient high volume patterning technique to define the channel length. 
     2. Description of Related Art 
     Organic MOS transistors are similar to silicon metal-oxide-semiconductor transistors in operation. The major difference in construction is that the organic MOS transistor utilizes a thin layer of a semiconducting organic polymer film to act as the semiconductor of the device, as opposed to a silicon layer as used in the more typical in-organic silicon MOS device. 
     Referring now to  FIG. 1 , a cross-sectional diagram of a bottom-gate bottom-contact organic MOS transistor  100  is shown. A metallic region  122  is deposited on an insulating substrate  112  forming the gate  122  of the organic MOS device  100 . A thin dielectric region  120  is placed on top of gate region  122  to electrically isolate it from other layers and to act as the MOS gate insulator. Metallic conductors  118  and  116  are formed on the dielectric region  120  above the gate region  122  such that there is a gap  124  between conductors  116  and  118  overlapping gate metal  122 . The gap  124  is known as the channel region of transistor  100 . A thin film of organic semiconducting material  114  is deposited on dielectric region  120  and over at least a portion of metallic conductors  116  and  118 . A voltage applied between the gate  122  and the source  118  modifies the resistance of the organic semiconductor film  114  in gap region  124  in the vicinity of the interface between semiconductor region  124  and dielectric  120 . This is defined as the “field effect”. When another voltage is applied between the source  118  and the drain  116 , a current flows between the drain and source with a value dependent on both the gate-to-source and the drain-to-source voltages. 
     An organic transistor  200  can also be constructed as a bottom-gate top-contact structure as shown in  FIG. 2 . Conductor layer  222  is deposited and patterned on substrate  212 . A dielectric layer  220  is deposited on conductor layer  222 . A thin film of semiconductor material  214  is deposited on top of dielectric layer  220 . A conductive film is deposited and patterned on top of organic semiconductor  214  to form conductive source and drain regions  216  and  218 , such that there is a gap  224  that overlaps the underlying gate metal layer  222 . The gap  224  is known as the channel region of transistor  200 . Through the field effect, a voltage is applied between gate conductor  222  and source  218  that modifies the resistance of the organic semiconductor  214  in the gap region  224  in the vicinity of the interface between the semiconductor region  214  and the dielectric  220 . When another voltage is applied between source  218  and drain  216 , a current flows between the drain and the source with a value dependence on both the gate-to-source and the drain-to-source voltages. 
     Organic transistor  300  can also be constructed as a top gate structure as shown in  FIG. 3 . A conductive film is deposited and patterned on an insulating substrate  312  to form conductive regions  318  and  316 . One of these conductive regions is known as the source  318 , and the other as the drain  316 . The gap  324  between source  318  and drain  316  is the channel region of transistor  300 . A thin organic semiconductor layer  326  is deposited on top of these conductive regions such that the entire gap  324  and at least a portion of conductive regions source  318  and drain  316  are covered. A dielectric layer  320  is deposited on top of semiconductor layer  326 . A conductive layer  322  is deposited and patterned such that at the underlying gap  324  and at least a portion of the source  316  and the drain  316  are covered. The field effect causes the resistance of the organic semiconductor  326  inside the gap  324  in the vicinity of the interface between the semiconductor  326  and the dielectric  320  to decrease as a voltage is applied between the gate  322  and the source  318 . When another voltage is applied between the source  318  and the drain  316 , current flows between the source  318  and the drain  316 . The value of the current depends on the voltage between gate  322  and the source  318 . 
     In all of the structures shown in  FIGS. 1–3 , all layers may be patterned as long as the gate conductor overlaps the channel region gap and at least a portion of the source and drain, and organic semiconductor and dielectric are placed so that the gate conductor and the source/drain conductor are electrically isolated. 
     Organic semiconductor materials are often classified as polymeric, low molecular weight, or hybrid. Pentacene, hexithiphene, TPD, and PBD are examples of low weight molecules. Polythiophene, parathenylene vinylene, and polyphenylene ethylene are examples of polymeric semiconductors. Polyvinyl carbazole is an example of a hybrid matrial. These materials are not classified as insulators or conductors. Organic semiconductors behave in a manner that can be described in terms analogous to the band theory in inorganic semiconductors. However, the actual mechanics giving rise to charge carriers in organic semiconductors are substantially different from inorganic semiconductors. In inorganic semiconductors, such as silicon, carriers are generated by introducing atoms of different valences into a host crystal lattice, the quantity of which is described by the number of carriers that are injected into the conduction band, and the motion of which can be described by a wave vector k. In organic semiconductors, carriers are generated in certain materials by the hybridization of carbon molecules in which weakly bonded electrons, called π electrons, become delocalized and travel relatively far distances from the atom which originally gave rise to that electron. This effect is particularly noted in materials comprising of conjugated molecules or benzene ring structures. Because of the delocalization, these π electrons can be loosely described as being in a conduction band. This mechanism gives rise to a low charge mobility, a measure describing the speed with which these carriers can move through the semiconductor, resulting in dramatically lower current characteristics of organic semiconductors in comparison to inorganic semiconductors. 
     Though organic transistors have much lower performance than inorganic transistors, the materials and processing techniques to produce organic transistors cost significantly less than those used to produce inorganic transistors. Therefore, organic transistor technology has application where low cost is desired and low performance is acceptable. As the effective performance of an organic transistor is increased, the number of applications for organic transistor technology also increases. An example of this type of application is Radio Frequency Identification (RFID) tags. Though RFID tags can be produced that operate at any frequency, it is desirable to produce RFID tag using frequency ranges that are used in typical applications. One such typical frequency for RFID tags is 13.56 Mhz, a frequency that is well above the unity gain frequency of organic transistors, and in the range where non-quasi-static behavior needs to be taken into account. 
     The performance of an organic transistor is proportional to the charge mobility divided by the channel length squared. Therefore, the channel length, the gap between the source and drain, is a very strong function of the performance of an organic transistor. With a manufacturable technology capable of more finely defining this channel length, the performance of the organic transistor is greatly enhanced. 
     Conventional techniques for defining features during organic field effect transistor (OFET) fabrication have typically involved the use of photolithography and vacuum deposition. However, such methods do not result in the low price points that are desired. Though photolithography can define fine channel length dimensions, these techniques are not practically applied to the construction of low-cost organic technology due to cost of the many steps involved-photo resist spin, photo resist expose, photo resist develop, etch, photo resist removal, and clean. Some more efficient techniques for printing fine resolutions have been explored, such as microcontact printing (μ-CP). With this technique, smaller feature sizes have been demonstrated. However, μ-CP has not been shown to be compatible with large-scale production due to fragile print stamps, alignment issues, and poor throughput. Another approach has involved pre-patterning the substrate before printing where a photomask may be used to define a hydrophobic region within a channel with surrounding hydrophilic regions. The modified wetting properties of the substrate generally afford a narrower feature size after printing than would otherwise be achievable without the use of such feature-enhancing techniques. While this method has been demonstrated in certain laboratory settings, it is uncertain whether this process can be made cost effective for mass production. Notwithstanding the preceding, a method of forming sufficiently narrow channels that is compatible with existing manufacturing processes is desirable for improved or otherwise suitable OFET performance. 
     SUMMARY OF INVENTION 
     According to the present invention, laser ablation is utilized to define the channel length of an organic transistor. In an embodiment of this invention, a substrate is coated with a deposition of a metal or conductive polymer deposition, applied in a thin layer in order to enhance the resolution that can be attained by laser ablation. 
     According to an embodiment of the present invention, the laser ablation technique can be used in a roll-to-roll process, and even though it is a subtractive technique as opposed to additive printing techniques, it does have the potential to achieve speeds, volumes, prices and resolutions that are adequate to produce printed electronic technologies. Laser ablation is removal of material through the use of light. The ability of the material to absorb laser energy limits the depth to which that energy can perform useful ablation. Ablation depth is determined by the absorption depth of the material and the heat of vaporization of the work material. The depth is also a function of beam energy density, the laser pulse duration, and the laser wavelength. Laser energy per unit area on the work material is measured in terms of the energy fluence. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not by limitation in the accompanying figures in which like reference numerals indicate similar elements and in which: 
         FIGS. 1–3  are cross-sectional views of an inorganic MOS transistor including an insulating substrate, organic polymer film, dielectric layer, and conductive gate according to the prior art; 
         FIG. 4  illustrates a laser ablation system according to the present invention to pattern a metal layer of a top gate transistor structure; 
         FIG. 5  illustrates a laser ablation system according to the present invention to pattern a metal layer of a bottom gate, bottom contact transistor structure; 
         FIG. 6  illustrates a laser ablation system according to the present invention to pattern a metal layer of a bottom gate, top contact transistor structure; 
         FIG. 7  illustrates a pattern of RFID tags and antennas on a substrate during production; 
         FIG. 8  illustrates a method by which laser ablation is utilized in high volume production of printed RFID tags; 
         FIG. 9  illustrates another method by which RFID tags can be produced by a continuous roll-to-roll web; 
         FIG. 10  illustrates a method by which the layer to be ablated can be better distinguished from the layer underneath it; 
         FIG. 11  illustrates the use of a laser ablation system to form holes through the layers of dielectric and semiconductor to form holes which serve as an interconnect between metal layers; and 
         FIG. 12  is a laser ablation system including a plurality of excimer lasers according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 4 , a system for laser ablation of a metal layer of a top gate organic transistor is shown. The light from an excimer laser  401  is appropriately blocked by photomask  402 , and impacts the metal layer  403  on insulating substrate  404  where there is an opening in the photomask  402 . The light energy is of appropriate wavelength, absorption depth, pulse width, and repetition rate as to vaporize the metal where the light impacts on the metal. Laser ablation can typically ablate line width of 10 μm and higher. In one embodiment of this invention, the metal layer  404  is made unusually thin, 20 nm to 60 nm, in order to attain a capability to ablate much finer lines, in the order of 2 μm to 4 μm. 
       FIG. 5  shows the same system described above when applied to the definition of the channel on a bottom gate, bottom contact device. In this case, the metal layer  518  being ablated is on top of dielectric layer  520 . 
       FIG. 6  shows the same system described above when applied to the definition of the channel on a bottom gate, top contact device. In this case, the metal layer  618  being ablated is on top of semiconductor layer  614 . 
     Though the above figures show only one opening of the photomask, actually the patterning of all metal within the exposure area of the laser ablation system is patterned all at once. In this sense, laser ablation is a subtractive process, requiring that the substrate is coated with the material of choice, and is then removed in appropriate areas by laser ablation as defined by a mask. In one embodiment, a large substrate is covered with a material of choice and with a single ablation exposure defined multiple RFID tags and antennas to create a pattern as shown in  FIG. 7 . Substrate  700  consists of a multiplicity of RFID tags  701 , each circuit surrounded by a printed antenna metal  702 . It should be noted that the exposure area that a laser system can ablate depends on the size of the optics as well as the total energy needed to ablate the material. Therefore, the RFID circuit must be kept to a dimension in order to accommodate the optics of the laser system, and the conductive material should be chosen to have low ablation energy. The metal layer can be exposed multiple times in order to accumulate the amount of energy needed for the ablation to be complete. 
       FIG. 8  illustrates a method by which laser ablation is utilized in high volume production of printed RFID tags. In this embodiment, the need to pattern the antenna metal by ablation is removed, thereby providing two advantages. The first advantage is that the laser light is concentrated on a smaller area, thereby making it possible to increase the ablation energy per unit area and enabling the ablation of a wider selection of materials. The second advantage is the material cost savings produced by reducing how much metal is removed. In this embodiment, a substrate  800  has separate rectangles  801 ,  802 , and  803  of a continuous layer of metal in those areas where individual RFID tag circuits are to be formed. These rectangles can be produced by printing conductive polymer, nano-particle metal ink, another conductive ink formulation. Alternatively, this metal could be deposited by a masked evaporative or sputtered metal system. The pattern of metal rectangles is repeated across the entire substrate where the circuit of the RFID tag is to be formed. A space between the metal rectangles is designed to accommodate a printed metal antenna  704  around each circuit at a later time. In another embodiment, the antenna coil is printed before laser ablation. 
     Substrate  800  represents either an individual sheet or part of a roll-to-roll web. In the case of an individual sheet, the laser head steps and repeats over each rectangle, thereby aligning the laser head to expose each rectangle individually, thereby patterning all transistors of one RFID tag at a time. The same method can be utilized when substrate  800  is a roll-to-roll web. However, in this circumstance, the web would need to stop as the laser head steps across the web. It would be desirable if a single exposure can accommodate the entire width of the web, therefore allowing the web to move continuously in the direction of the web. This can be accomplished by using a system with multiple laser heads, one head for each RFID circuit across the web. These laser heads then fire at the same time, ablating all RFID tags along the width of the web. 
     Such a laser ablation system  1200  can be seen in  FIG. 12 , which includes excimer lasers  1202 ,  1204 , and  1206 , corresponding to metal rectangles  801 ,  802 , and  803 . Excimer lasers  1202 ,  1204 , and  1206  are placed above metal squares on substrate  1210 . Masks  1212 ,  1214 , and  1216  are used to direct the laser light to pattern each metal square. The excimer lasers  1202 ,  1204 , and  1206  are aligned with masks  1212 ,  1214 , and  1216  and the metal squares below on substrate  1210 . There is one mask for each laser across the web. These masks are used to pattern the laser light onto the corresponding square, thereby performing a laser ablation on the metal square. It should be noted that the diagrams of  FIGS. 4 ,  5 , and  6  illustrate how an opening on the single mask blocks the light in the non-image areas, whereas in  FIG. 12 , the entire mask is shown. Masks  1212 ,  1214 , and  1216  have openings corresponding to areas where ablation is desired on the metal squares on substrate  1210 . While three excimer lasers are shown, additional excimer lasers can be used if desired such that more RFID tags can be accommodated in a single row across the web. 
       FIG. 9  illustrates another method by which RFID tags can be produced by a continuous roll-to-roll web. In this embodiment, web  900  is made sufficiently narrow to accommodate only a single RFID tag. A metal rectangle  901  is deposited where the circuit of the RFID is to be formed. Sufficient space is permitted between metal squares and in the width of the web to allow the printing of an antenna around the circuit at a later step. 
       FIG. 10  illustrates a method by which the layer to be ablated  1003  can be better distinguished from the layer  1004  underneath it. Such distinction makes it possible to ablate the layer to be ablated, but leave the layer underneath unaltered. The layer  1003  is different, depending on which organic transistor structure is being fabricated. In this embodiment, another layer of material  1005  is added which is electrically benign. This material has a higher ablation energy than the conductor being ablated, and should be an insulator in order to remain electrically benign. In another embodiment, the layer  1004  is treated with a chemical to harden the surface or to increase its reflectivity in order to become more resistant towards laser ablation of the layer  1003  above it. 
     Referring now to  FIG. 11 , structure  1100  illustrates the use of laser ablation in order to form a hole  1130  through dielectric  1120  and semiconductor  1126  to make contact with metal pad  1116 . In a subsequent step, metal is deposited thereby connecting metal pad  1116  with the metal above it to form a method of providing interconnect between the two metal layers. Alternatively, this hole could be formed by leaving a hole in the dielectric and the semiconductor when those layers are printed. However, the resolution of the print process results in a much larger hole  1130  than if it is laser ablated. In addition, the dielectric must generally be printed with a hole even larger than the resolution would allow since an alignment tolerance must be taken into account to ensure that the printed dielectric hole overlaps the hole printed in the organic semiconductor. 
     While the invention has been described in detail in the foregoing description and illustrative embodiment, it will be appreciated by those skilled in the art that many variations may be made without departing from the spirit and scope of the invention. Thus, it may be understood, for example, that the structures above could include self-assembled monolayers (SAMs), corona treatment, or other surface treatments to obtain desired surface energy and contact angles for optimized print characteristics. The metal layers may contain another conductive layer between the source/drain or gate layers and the surface upon which it is printed in order to promote enhanced adhesion, to increase or decrease wetting of the print surface. Metal layers may be treated with gold immersion or thiol processing to reduce oxidation, increase the effective work function of the metal, and promote desired alignment of the semiconductor polymer and crystalline structures. Various curing steps either at each deposition step or at the end of the entire process may also be included.