Abstract:
A method of forming an organic semiconductor includes forming a thin film transistor (“TFT”) backplane; forming a pixel well over the TFT backplane using a photoresist; performing a first plasma etch of the pixel well; stripping the photoresist in the pixel well; performing a second plasma etch of the pixel well; performing a first wash of the pixel well; exposing the pixel well to ultraviolet light; performing a second wash of the pixel well; and forming an organic photodiode in the pixel well.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application relates to and claims priority of U.S. provisional patent application (“Copending Provisional Application”), Ser. No. 62/361,341, filed on Jul. 12, 2016. The disclosure of the Copending Provisional Application is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to image sensor circuits including organic photodiodes, and, more particularly, to optimizing the processing steps used in manufacturing these image sensor circuits. 
       BACKGROUND OF THE INVENTION 
       [0003]    Organic semiconductors, including image sensor circuits, have many potential advantages over traditional amorphous silicon-based semiconductors. The chemicals used in the photodiode for the image sensor circuit can be tailored to be sensitive to different frequencies ranging from ultraviolet to infrared. The chemical films have lower stress and lower process temperatures; thus, working well with lower temperature applications, such as flexible substrates. 
         [0004]    Manufacturing of individual organic photodiodes on a substrate to create photo sensing arrays remains a challenge. The thin film transistor (“TFT”) backplane upon which the organic photodiode is deposited must be free of any organic or inorganic residue, such as residual photoresist, etch process compounds and must be very planar. This is especially important when making a photo sensitive array. If the surface is not free of any residue, then the organic chemical will not have sufficient electrical contact with the bottom electrode of the TFT backplane. This will cause non-repeatable performance from the photo sensing array from one process run to another. If the TFT backplane electrode is not planar, then the organic material may not completely cover the electrode and possible electrical shorts will result. Even if the organic material is able to cover the uneven electrode surface, performance of the organic material will be inconsistent and unrepeatable. 
         [0005]    What is desired is an improved process of cleaning and optimizing the surface of the TFT backplane to ensure repeatable performance of an organic semiconductor. 
       SUMMARY OF THE INVENTION 
       [0006]    The method of the present invention provides a consistent surface in a pixel well upon which an organic semiconductor photodiode will be manufactured, so that the interface between the pixel well and the organic photodiode can be optimized and the performance of the resultant image sensor circuit can also be optimized. 
         [0007]    Optimizing the interface between the bottom contact of a TFT backplane and an organic semiconductor is achieved by a two-stage multistep process comprising lithographic, dry etch, chemical clean, dry etch cleaning processes. The second stage comprises several steps to clean surface contamination before beginning to manufacture an organic semiconductor. 
         [0008]    A first method of forming an organic semiconductor according to the present invention comprises forming a thin film transistor backplane; forming a pixel well over the TFT backplane using a photoresist; performing a first plasma etch of the pixel well; stripping the photoresist in the pixel well; performing a second plasma etch of the pixel well; performing a first wash of the pixel well; exposing the pixel well to ultraviolet light; performing a second wash of the pixel well; and forming an organic photodiode in the pixel well. 
         [0009]    A second method of forming an organic semiconductor according to the present invention comprises forming a thin film transistor backplane; forming a pixel well over the TFT backplane using a photoresist; performing a plasma etch of the pixel well; stripping the photoresist in the pixel well; performing a residue rinse of the pixel well; performing a first wash of the pixel well; exposing the pixel well to ultraviolet light; performing a second wash of the pixel well; and forming an organic photodiode in the pixel well. 
         [0010]    The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of an embodiment of the invention as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1-11  are cross-sectional drawings of processing steps for a manufacturing method according to present invention; and 
           [0012]      FIG. 12  is a cross-sectional drawing of a substitute processing step that can be used in the manufacturing method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    During the processing steps of making the bottom contact of the TFT backplane, polymer residues can contaminate the surface of the contact metal, such as ITO, chrome, or any metal oxides having the formula A x B x C x O x . This can typically occur during the lithography masking step when some residual photoresist does not get completely removed before the etching step. The residual resist will form an etch block on the contact surface and result in uneven etching across the contact pad. This manifest itself in microscopic pillars on the surface of the bottom contact pad. This can cause electrical shorting or degraded diode performance of the organic photodiode. 
         [0014]    The other form of contamination that can occur on the backplane bottom contact surface is polymer re-deposition during the dry etch process. This contamination can manifest itself as tiny particles dispersed on the sidewall of a pixel well or on the surface of the bottom contact. An example of these particles can be manifested as a plurality of raised regions or hillocks having a sub-micron feature size. 
         [0015]    These particles can impede good or sufficient electrical contact between the organic photodiode materials and the bottom contact of the TFT backplane. This will cause electrical shorts or degraded performance of the organic diode pixel or pixels in a photo sensor array. 
         [0016]    The method of the present invention resolves this problem by applying two methods of surface cleaning or optimization before processing the organic semiconducting materials onto the bottom contact of the TFT backplane. The first step in the optimization process is ensuring that the pixel well is properly cleared of photoresist. This can be accomplished in three ways, either by increasing the photoresist to a higher dose than nominal, such as 2.5× times the minimum dose to clear with a particular develop time. The other method is to apply a longer develop time with nominal exposure dose. The third method is to apply both, an increase exposure dose as well as longer develop time. This will allow the etch process to etch evenly across the bottom contact pad of the TFT backplane. 
         [0017]    Referring now to  FIG. 1 , a cross-sectional view of a finished TFT backplane is shown including a glass (or other) substrate  102 , metal lines  104 , and a TFT including a gate  103  and an island  105 . An ILD (Inter Layer Dielectric) layer  106  is also shown. A metal stack for forming a shield and for coupling metal lines  104  to the organic photodiode (not shown in  FIG. 1 ) includes a first TiW (Titanium Tungsten) layer  108 , an Al (Aluminum) layer  110 , a second TiW (Titanium Tungsten) layer  112 , and an ITO (Indium Tin Oxide) layer  114 . 
         [0018]    Referring now to  FIG. 2 , a bank layer  116  is deposited on the surface of the ILD  106  layer  106  as well using a solution process (i.e. extrusion, slot die, spin coating spray coating, or inkjet techniques). The bank layer  116  ideally has a thickness of between 1.0 and 6.0 microns. The bank layer  116  is deposited at ambient temperature followed by a soft bake (to remove solvents) at T=50° C. to 100° C. The bank layer  116  materials may include, but are not limited to, Dow Chemical Cyclotene 6100 series, or variants thereof, Honeywell PTS series, Microchem SU-8, TOK TPIR PN-0371D or other material common in the art. The bank layer  116  material provides excellent planarization (&gt;90%) over the entire substrate. 
         [0019]    Referring now to  FIG. 3 , the bank layer  116  material is cured (T=200° C. to 300° C.; ideally less than 250° C.) in an N 2  atmosphere for 1-6 hours followed by coating with a photoresist layer  118 . The photoresist layer  118  can be a positive-toned, novalac-based material. The typical thickness of the material is between 2.0 and 8.0 microns and it is deposited using an extrusion coater, or slit coater. 
         [0020]    Referring now to  FIG. 4 , the photoresist layer  118  is exposed by i-line exposure and developed to form individual pixel wells down to the bank layer  116 . An exposed portion  120  of the photoresist layer  118  that will form the pixel well is shown in  FIG. 4 . A typical dose range for exposing the photoresist layer  118  is 75 mJ to 200 mJ, and is ideally 90 to 125 mJ. The thickness of the photoresist layer is between one and ten microns, and ideally between two and five microns. 
         [0021]    Referring now to  FIG. 5 , in order to minimize polymer redeposition occurrence during a dry etch step, a gas mixture of 200-1000 SCCMs of C x F y  (fluorocarbons) with 50-500 SCCMs (standard cubic centimeter per minute) of O 2  (Oxygen) is used. Another option according to the method of the present invention to remove any polymer redeposition is to add an additional gas mixture of 50-500 SCCMs of each chlorine (Cl 2 ) gas, Boron Chloride (BCL 3 ), and Argon gas. This dry etch step is achieved by using an RF, microwave plasma etcher with a top power between 3000 W-800 W and Bias power between 500 W-2500 Watts. The plasma used in the dry etch step is shown as layer  122  in  FIG. 5 . 
         [0022]    Referring to  FIG. 6 , the third step of the process according to the present invention is to process the TFT backplane through an organic film remover, shown as layer  124 . The organic film remover is typically an NMP-based material (N-Methyl-2-Pyrrolidone). The wash is usually performed at 60° C. for five minutes followed by a de-ionized water rinse. The bank layer  116  material is immune to the strip chemistry and remains as part of the device. Following the strip process, the individual pixel wells are formed to create the organic photodiode layer in the process (not shown in  FIG. 6 ). It is important to note that the bank layer  116  material is highly planar, thus providing excellent dielectric separation between the metal layer features. The bank layer  116  can be rinsed multiple times to ensure complete removal of any photoresist polymer. 
         [0023]    Referring now to  FIG. 7 , the fourth step of optimizing the TFT backplane bottom contact surface according to the method of the present invention is to process the plates through a dry etch cleaning step. The first option is to use a gas mixture of 200-1000 SCCMs of C x F y  with 50-500 SCCMs of O 2  and 50-500 SCCMs of each chlorine (Cl 2 ) gas. Boron Chloride (BCL 3 ), and Argon gas. The top power of the RF plasma dry etcher is between 3000 W-8000 W and the bias power is between 500 W-2500 W. The plasma used in the dry etch step is shown as layer  126  in  FIG. 7 . 
         [0024]    Referring now to  FIG. 8 , the fifth step according to the method of the present invention is to wash the plates in de-ionized water followed by a drying step. The water rinse is shown as layer  128  in  FIG. 8   
         [0025]    Referring now to  FIG. 9 , the sixth step according to the method of the present invention is to expose the array to UV light  130  between 30-300 seconds to remove any microscopic organic contamination buildup from packaging or handling. 
         [0026]    Referring now to  FIG. 10 , the seventh, and last, step according to the method of the present invention is to wash the TFT backplane with water followed by a drying step. This is performed just before manufacturing the organic photo sensor array. The water rinse is shown as layer  132  in  FIG. 10 . 
         [0027]    Referring now to  FIG. 11 , the completed pixel cell substantially free of contamination is shown except for forming the organic photodiode, which is placed into pixel well  134 . An appropriate organic photodiode structure can be found is co-pending patent applications US 2017/0054097, US 2017/0084665, Ser. No. 15/344,335, Ser. No. 15/369,455, and Ser. No. 15/381,679, which are all incorporated herein in their entirety. 
         [0028]    Referring now to  FIG. 12 , a cross-sectional drawing of a substitute processing step that can be used in the manufacturing method according to the present invention is shown. Specifically,  FIG. 12  can be substituted for  FIG. 7  in the sequence of steps shown in  FIGS. 1-11 , previously discussed. A second method of optimizing the bottom contact of a TFT backplane before manufacturing an organic photo sensor array according to the present invention is to complete the steps described with respect to  FIGS. 1-6  above, but instead of applying a dry etch cleaning step, a polymer residue removal step is performed as is shown in  FIG. 7 . This is a wet etch step that rinses the array between 60 and 300 seconds at room temperature. The chemical used comprises primarily ammonium fluoride, hydrofluoric acid and ethylene glycol. Afterwards, the remaining steps associated with  FIGS. 8-12  are completed to finish the process. 
         [0029]    The identification numerals are changed slightly in  FIG. 12  to show a glass or other substrate  202 , island  203 , metal lines  204 , island  205 , ILD layer  206 , metal stack  208 ,  210 ,  212 ,  214 , bank layer  216 , and polymer residue rinse  225 . All of the layers except for the polymer residue rinse were described previously. 
         [0030]    While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention.