Patent Publication Number: US-9406896-B2

Title: Pre-fabricated substrate for printed electronic devices

Description:
BACKGROUND 
     Solution based all-additive printing process enables low cost fabrication of electronic devices on a large area flexible substrate. These printing processes offer several advantages, including fast prototyping with on-demand custom device, patterning devices at low temperature, and applies to a broad range of applications for electronic device manufacture. 
     Demand continues to drive improvements to fabricate faster, smaller and lower cost devices with higher integrated circuit density. Many of these printing processes use organic semiconductors. Organic thin-film transistors (OTFT) have low electron or hole mobility. Because of this low mobility, the desired device performance requires a large ratio of the TFT channel width to channel length (W/L). 
       FIG. 1  shows an example of an inverter  10  manufactured from current ink jetting techniques. Conventional ink jetting technology produces a line  12  having a minimum line width of about 60 micrometers for channel  14 , with the minimum channel length being about 30 micrometers. The large W/L ratio is realized by a large channel width of about 2 millimeters for the TFT channel, resulting in a W/L ratio of approximately 67 (2000/30). The resulting circuit size therefore has a larger than desirable size. 
     The transistor current in the linear or saturation region is proportional to the W/L ratio or the square of the W/L ratio, minimizing the channel length allows for higher currents. In addition to the challenge of minimizing the channel length, the wide width of the conductive lines or traces causes a large area of the overlap between source/drain ( 11 / 12 ) and gate  16  of the transistor made by a conventional jetting technique. In one embodiment, the overall area is approximately 9×10 4  micrometers 2 . Minimizing the overlap area will result in reducing the overlap parasitic capacitance and improving the device speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art embodiment of a printed electronic circuit. 
         FIG. 2  shows an embodiment of a pre-patterned circuit substrate. 
         FIG. 3  shows a planar view of staggered parallel lines on a pre-patterned substrate. 
         FIG. 4  shows an embodiment of an electronic device manufactured on a pre-patterned substrate. 
         FIG. 5-7  show embodiments of variations of parallel line structures on a pre-patterned substrate. 
         FIG. 8  shows an embodiment of a parallel line structure having floating lines and an equivalent circuit. 
         FIGS. 9-11  show embodiments of pre-patterned line structures on a substrate. 
         FIG. 12  shows an embodiment of a printing system operable to jet conductive inks over capillary channels. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 2  shows an embodiment of a pre-patterned or pre-fabricated substrate usable for printing electronic devices. The substrate enables an additive printing process to produce desired devices and circuit patterns with critical dimensions much smaller than the limitations of conventional printing process. The generically designed pre-patterned substrate comprises of periodic segments of repetitive patterns of interdigitated line structures in micrometer or nanometer scale. The process utilizes these patterns to form transistor channels by printing semiconductor inks over the fine features without a critical alignment with the substrate. The printing layout design determines the location of a thin film transistor (TFT) and the channel width. 
     The term ‘channel length’ is the distance between the source and the drain. The parallel lines are staggered such that they extend out at each end to allow for contact. The extension length may range from a few micrometers to a few hundred micrometers. As will be seen later, the alternating lines each form a source contact at one end and a drain contact at the other. The distance between these alternating lines in the X direction makes up the channel length. 
     The ‘channel width’ is defined as the sum of each channel width inside an area of semiconductor ink, running in the y direction. The number of the channels will be determined by conductive patterns formed by a subsequent jetting process that bundles some of the lines. As will be seen in further embodiments, the dimension of these channels may vary in the y direction and several channels will typically be included inside the area of the ink drop(s). 
     In the embodiment of  FIG. 2 , the substrate  20  may consist of a supporting material  24  such as various types of plastics, glass substrates, silicon wafers or steel foils. The substrate may optionally include an insulating buffer layer such as  22 . A series of segments such as  26  includes a set of pre-patterned lines such as  28  and  30 . The lines may consist of conductive materials such as silver, gold, platinum, or aluminum. As mentioned above, the interdigitated lines  28  and  30  are staggered such that the contacts for the lines staggered with  28  are to one side of the segment, and the contacts for the lines staggered with  30  are to the other side of the segment. An isolation region resides between the segments to allow the contacts for the lines to be printed. The dimension of the isolation region in the Y direction may range from a few tens of micrometers to a few millimeters. 
       FIG. 3  shows a more detail view of one segment having a semiconductor ink drop  38  that forms a thin film transistor according to one embodiment. In general, multiple overlapping ink drops may be used to form a semiconductor region for the transistor. The source contacts  34  and the drain contacts  36  are bundled into one thin film transistor. The source contacts  34  consist of those lines that are staggered with line  28 , the drain contacts consist of those lines that are staggered with line  30 . The spacing between each staggered line, which comprises the channel length, may range from a few nanometers to tens of micrometers, such as from 1 nanometer to 10 micrometers. In the example of  FIG. 3 , the channel length is 5 micrometers. The line width may range from a few nanometers to hundreds of micrometers, such as from 1 nanometer to 100 micrometers. The type of conducting material use and the requirement of the conductance for a given current and voltage determines the line width. For example, for a silver line of 500 micrometer in length, 3 micrometers wide and 200 nanometers thick, the resistance is about 13 ohms. This is sufficient for loading 1 milliamp current under 5 volts of source-drain voltage. 
     In  FIG. 3 , the dimension of the segment and staggered region is slightly larger than the drop size of the semiconductor pattern. The jetting size of one embodiment of current p-type organic semiconductor has a diameter of about 250 micrometers, and the segment has a dimension of about 400 micrometers. In this embodiment, the drop covers about 30 channels in the circular area. Each channel has a different channel width which is in the y direction. In this particular embodiment, the total channel width is about 6 millimeters. This particular segment has the spacing between the source lines and the drain lines of 5 micrometers and the line width is 3 um. This results in one drop having the maximum W/L ratio of 1200, (6000/5). This method creates a circuit having more than 10 times the W/L ratio compared with the printed source-drain approach. The resulting circuit can handle currents more than 10 times than previous circuits in the linear transistor operation condition. 
     In addition to the advantage of reducing the TFT channel length, the overlap capacitance between the source-drain and the gate electrode is also reduced due to the narrow line width of the pre-patterned feature. For a line width of 3 micrometers, the overlap area reduces by about 20 times, when compared with 60 micrometers wide traces made by convention ink printing. This reduction further improves the circuit performance. 
       FIG. 4  shows an application of making a clock circuit by ink jet printing processes on the universally pre-patterned substrate. Patterns of source-drain connection traces are formed by silver ink jetting on the substrate (in gray). These patterns not only serve as connections between organic TFTs at the lower source-drain level, but also define the TFT channel location and channel width by bundling some of the leads on both sides of a segment. The length of the leads has a range of a few micrometers to a few hundred micrometers, large enough to allow tolerance of printing resolution for the ink jet to place the conductive traces to connect the number of leads. In this example, p-type TFTs are formed from p-type ink such as  42  shown at the top of the circuit and n-type TFTs reside on the bottom formed from n-type ink such as  44 . The inverter  40  compares to that in  FIG. 1 . The isolation regions between the segments allow printing of the conductive traces to connect various TFT source and drain contacts. The rest of the processes include gate dielectric coating, via formation for vertical interconnection, and gate level patterning are similar to our current conventional processes. 
     As shown in  FIG. 4 , the gate of transistor  42  and  44  are connected through area  46 . Since these parallel lines in the segment are isolated when they are not bundled, the gate lines passing through this area do not overlap with the source drain electrode and therefore do not contribute to the overlap parasitic capacitance. The via  47  allows for source/drain bottom metal to the gate top metal connection. Wherever the area where the lines are not bundled can be used for various connections which make the universally patterned substrate reconfigurable for various types of circuits. If required, these areas can be used for vertical or lateral metal connections. 
     Many modifications and variations exist for the embodiments disclosed here.  FIGS. 5 through 7  show possible variations in the segments. The length of a segment may differ from its neighboring segment as shown by segments  50 ,  52 ,  54  and  56 . In addition, the line spacing, which defines the channel length, may vary from segment to segment. As shown in  FIG. 6 , the segments  50  have one line spacing and the segment  58  has a tighter spacing. In one embodiment the segments  50  have spacing of 5 micrometers and segment  58  has a spacing of 3 micrometers.  FIG. 7  shows a spacing variation between segment  50  spacing and segment  58  spacing, but within one segment. 
     Other variations include different conducting materials or treatments, or varying line lengths corresponding to channel widths. The width of the sub-segment is also determined by the width of the coverage of ink drops, which can also be controlled to provide more flexibility. 
     In another embodiment, the patterns may include floating lines inserted between each pair of staggered lines, shown in  FIG. 8 . The floating line such as  60  may reside between the source and drain lines of each pair. This may allow multiple TFTs to be connected serially. It may also reduce the source-drain leakage current.  FIG. 8  also shows the equivalent circuit  62 . 
     Other variations exist within the structures of the patterned lines. In one embodiment, shown in  FIG. 9 , conductive traces such as  70  reside on the surface of the substrate. Metal or indium-tin-oxide deposition followed by imprint lithography may form these lines, or photolithographic patterning and etching. Alternatively, the lines could result from recessed channels such as  72  in  FIG. 10 . Imprint photolithography or photolithography followed by RIE etching may form these lines. The narrow channels may draw pre-cured conductive ink by capillary force as shown in  FIG. 12 . In  FIG. 12 , an ink jet  80  dispenses conductive ink  82  into the channels, which then draws the ink through capillary action to form the lines such as  84 . 
     In yet another embodiment of the parallel lines, they may result from self-assembled monolayers of organic molecules. The process deposits these molecules by stamping or laser patterning. The monolayer controls the surface energy and wetting regions of the solution ink. For example, with aqueous silver, the solution with stay in the hydrophilic region and de-wet away from the hydrophobic region, forming the conductive patterns. 
     In this manner, ink jetting technology can form smaller electronic devices with faster speeds and the ability to handle higher currents. The pre-patterned substrate has high flexibility and configurability and provides the opportunity for custom manufacturing of electronic circuits and devices at relatively low costs and faster manufacturing times. 
     It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.