Patent Publication Number: US-2017365718-A1

Title: Insulator/metal passivation of motft

Description:
FIELD OF THE INVENTION 
     This invention generally relates to passivation of metal oxide thin film transistors to improve environmental stability. 
     BACKGROUND OF THE INVENTION 
     Metal Oxide Thin Film Transistors (MOTFT) get a lot of interest because of their high performance, including high mobility, better positive/negative bias stress stability, and better current stress stability. But challenges remain in the area of environmental stability. 
     Many processing steps have to be performed after the fabrication of MOTFTs to incorporate the MOTFTs into displays or sensors. These processing steps may require moderate temperatures (&gt;200° C.) under all kinds of ambience (vacuum, inert, reducing or oxidizing, etc.). Furthermore, all electronic devices are sensitive to humidity, which they have to be protected against. 
     The best passivation film that protects against humidity is silicon nitride (SiN). The silicon nitride is typically deposited by low pressure chemical vapor deposition (LPCVD) at high temperature (&gt;400° C.). Silicon nitride can also be deposited by plasma enhanced chemical vapor deposition (PECVD) at lower temperature (&gt;200° C.). In the flat panel display industry, LPCVD is not available because of the high deposition temperature and PECVD is the only option. 
     The quality of PECVD SiN depends critically on the deposition temperature. The higher the deposition temperature is the better the quality of the SiN, i.e. as the deposition temperature is increased the film density, and thus the passivation quality, increases. However, a high mobility MOTFT can only survive a high temperature of approximately 200° C. in an oxidizing ambience because the MOTFT needs balancing internal oxygen vacancies with the ambience oxygen so that no further vacancies can be generated. The generation of additional oxygen vacancies (loss of oxygen) in the MOTFT will change the threshold voltage by moving the threshold voltage more negative. If the ambience is reducing (i.e. oxygen absorbing), the loss of oxygen from the metal oxide semiconductor material in the MOTFT will not stop. If the processing temperature is high, the diffusion through whatever material is on top of the metal oxide semiconductor material can be fast and the loss of oxygen fast enough to change the threshold voltage toward negative substantially. So MOTFT passivation becomes the major hurdle in the adoption by the flat panel display industry. The major passivation technique in the flat panel display industry today is the use of PECVD SiN. No flat panel display can be made without PECVD SiN passivation. To make MOTFT widely adopted by the TFT/flat panel industry, any passivation needs to be compatible with the standard passivation technology, namely PECVD SiN. 
     SiN passivation was originally adapted from the silicon industry into the amorphous silicon (a-Si) TFT industry. Because a-Si can withstand relatively high temperature, PECVD is very compatible with the a-Si TFT technology. But in the PECVD process of SiN, there are lots of hydrogen ions and molecules in the plasma. The ambience is not oxidizing, but can be reducing (removing or absorbing oxygen). The hydrogen ions or molecules can diffuse through whatever is on top of the metal oxide semiconductor material (e.g. partially completed SiN layer) at high temperature and reduce the metal oxide semiconductor material into metal ions. This reduction can move the threshold voltage negatively by a large amount. 
     It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art. 
     Accordingly, it is an object of the present invention to provide new and improved passivation of MOTFTs. 
     It is another object of the present invention to provide a new and improved process and material for passivation of MOTFTs that improves environmental stability. 
     It is another object of the present invention to provide a new and improved process and material for passivation of MOTFTs that is compatible with flat panel technology. 
     SUMMARY OF THE INVENTION 
     The desired objects of the instant invention are achieved in accordance with a method of fabricating a MOTFT including providing a metal oxide thin film transistor having a surface defined by spaced apart source/drain terminals positioned on a layer of semiconductor metal oxide and material in a space between the source/drain terminals, the space between the source/drain terminals defining a conduction channel in the layer of semiconductor metal oxide. The method further includes the steps of forming a layer of passivation material on the surface defined by the spaced apart source/drain terminals and the material in the space between the source/drain terminals, establishing oxygen vacancy equilibrium in the conduction channel of the layer of semiconductor metal oxide by annealing in an oxygen containing ambient the MOTFT and layer of passivation material and depositing at low temperature a layer including a noble metal, a refractory metal, and/or a transparent conducting metal oxide on the layer of passivation material overlying the space between the source/drain terminals. 
     The desired objects of the instant invention are also achieved in accordance with a specific embodiment thereof wherein a passivated metal oxide thin film transistor (MOTFT) is provided. The MOTFT includes a metal oxide thin film transistor having a surface defined by spaced apart source/drain terminals positioned on a layer of semiconductor metal oxide and material in a space between the source/drain terminals, the space between the source/drain terminals defining a conduction channel in the layer of semiconductor metal oxide. A layer of passivation material is positioned on the surface defined by the spaced apart source/drain terminals and the material in the space between the source/drain terminals and a layer including a noble metal, a refractory metal, and/or a transparent conducting metal oxide is positioned on the layer of passivation material overlying the space between the source/drain terminals. 
     The desired objects of the instant invention are also achieved in accordance with a method of fabricating a back-panel with a plurality of metal oxide thin film transistors for flat panel devices including one of a light emitting diode display, an image sensor array, a biosensor array, a pressure sensing array, an X-ray imager, or a touch sensing array. The method includes steps of providing a substrate, and fabricating on the substrate a plurality of passivated metal oxide thin film transistors, the fabrication of each passivated transistor including the steps of: 
     providing a metal oxide thin film transistor having a surface defined by spaced apart source/drain terminals positioned on a layer of semiconductor metal oxide and material in a space between the source/drain terminals, the space between the source/drain terminals defining a conduction channel in the layer of semiconductor metal oxide; 
     forming a layer of passivation material on the surface defined by the spaced apart source/drain terminals and the material in the space between the source/drain terminals; 
     establishing oxygen vacancy equilibrium in the conduction channel of the layer of semiconductor metal oxide by annealing in an oxygen containing ambient the metal oxide thin film transistor and layer of passivation material; and 
     depositing a layer including a noble metal, a refractory metal, and/or a transparent conducting metal oxide on the layer of passivation material overlying the space between the source/drain terminals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the drawings, in which: 
         FIG. 1  is a simplified layer diagram illustrating an etch-stop type of MOTFT with improved environmental stability passivation in accordance with the present invention; and 
         FIG. 2  is a simplified layer diagram illustrating a back channel etch (BCE) type of MOTFT with improved environmental stability passivation in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Turning to  FIG. 1 , an etch-stop type of MOTFT  10  is illustrated with passivation in accordance with the present invention. MOTFT  10  includes a substrate (support structure)  12 , which may be rigid, flexible, transparent, opaque, silicon, glass, plastic, stainless steel with a dielectric coating or any material capable of providing support for MOTFT  10 . A metal gate  14  is positioned on substrate  12  by any convenient means, preferably by non-critical patterning. Here it should be understood that while a single MOTFT is illustrated and described for convenience, MOTFT  10  could be part of a back-panel circuit for a flat panel display array, sensor array, etc. Thus, a large number of metal gates  14  may be simultaneously placed on substrate  12  and, since the placement is generally not critical, non-critical patterning can be used. 
     A thin gate insulator/dielectric layer  15  is deposited over gate  14 , preferably in a blanket deposition. A metal oxide film  16  is formed on gate dielectric layer  15  in overlying relationship to metal gate  14  and the surrounding area. Metal oxide film  16  is the semiconductor active layer for MOTFT  10 . Examples of metal oxides that can be used include zinc oxide (ZnO), indium oxide (InO), tin-oxide (SnO), gallium-oxide (GaO), indium zinc oxide (InZnO), tim-zink oxide (SnZnO), indium zinc gallium oxide (InZnGaO), indium tin zinc oxide (InSnZnO), indium aluminum zinc oxide (InAlZnO), indium silicon zinc oxide (InSiZnO), and similar materials or combinations thereof in blend, multiple layers or alloy forms. Also, the metal oxide semiconductor may be amorphous or polycrystalline, however, amorphous, or nanocrystalline with mean grain size substantially smaller than the TFT channel length (say, &lt;100 nm) is preferred. An etch-stop/passivation area  20  is patterned on metal oxide layer  16  and source/drain terminals  18  are formed on opposite sides of passivation area  20  on the upper surface of active layer  16 . The space between the source/drain terminals  18  defines the conduction channel for MOTFT  10 . Additional information as to preferred methods of fabrication of MOTFT  10  are available in U.S. Pat. No. 7,977,151, entitled “Double Self-Aligned Metal Oxide TFT”, issued Jul. 12, 2011, U.S. Pat. No. 8,679,905, entitled “Metal Oxide TFT with improved source/drain contacts”, issued Mar. 25, 2014, U.S. Pat. No. 9,356,156, issued on May 31, 2016, entitled “Stable High Mobility MOTFT and Fabrication at Low Temperature”, issued May 31, 2016. They are incorporated herein by reference. 
     Any passivation of a MOTFT requires three characteristics. First, the passivation material has to have very little chemical interaction with the metal oxide and with oxygen at subsequent processing temperatures and during useful lifetime. Any strong chemical interaction will move the threshold voltage. Second, the passivation material has to have very little diffusion for processing and ambient gases, such as hydrogen, oxygen, and water, at high temperature. Third, the material has to be electrically insulating from the metal oxide. The third requirement or constraint limits the choice of material to an insulator. 
     The insulator films that can be deposited on top of metal oxide without destroying the MOTFT are typically porous to oxygen and water. Examples are organic polymers (e.g. polyimide film from Toray Films or DuPont Teijin Films, acrylic films from JSR Corporation, Fuji Films or MicroChem, Bisbenzocyclobutene resins (BCB), SU8 film from MicroChem, poly(4-vinylphenol) (PVP) and its crosslinked version with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (PVP:HDA)) and PECVD SiO 2 , spin on insulators such as ZrO, sol gel and spin-on glasses, sputtered oxide (e.g. AlO, SiO 2 ). Once an insulator film of this type is deposited on the metal oxide, the structure can be annealed in the air at elevated temperature to restore the oxygen vacancy balance in the metal oxide active channel layer, if necessary. This restoration can be accomplished since oxygen diffuses through the insulator layers described under elevated temperature. 
     Turning again to  FIG. 1 , a passivation insulator film  22  is deposited over source/drain terminals  18  and passivation area  20  of MOTFT  10 . Passivation insulator film  22  is one of the more porous insulators such as an organic polymer film, a PECVD SiO 2  or SiON film, or any of the insulating materials described above. MOTFT  10  is then annealed back into oxygen vacancy equilibrium. 
     With insulator film  22  in place and metal oxide active layer  16  in oxygen vacancy equilibrium, a dense metal film  24  is deposited on insulator film  22  so as to cover and protect the space between source/drain terminals  18 , which defines the conduction channel for MOTFT  10 . For film  24  an inert metal, such as a noble metal (Au, Ag, Pd, Pt, etc.), a refractory metal (Mo. W, etc.) or any of the transparent metal oxides known in the semiconductor industry, such as ITO, SnO, AlSnO or AlZnO and the like, can be deposited by sputtering at low temperature. The inertness of the metal is manifested in its work function, generally being a high work function (&gt;4.5 eV). In at least some instances the noble or refractory metal can be replaced by a transparent metal oxide, such as ITO, SnO, AlSnO or AlZnO. In yet other instances, a stack metal film comprising multiple inert metals, refractory metals or transparent metal-oxides explained above can also be used. By putting a dense layer of inert metal or metal oxide on insulator film  22 , a composite film (polymer/metal or insulator/metal stack) is provided that is a good passivation film for MOTFTs (i.e. fulfills the three requirements listed above). 
     Turning to  FIG. 2 , a back channel etch type of MOTFT  10 ′ is illustrated with passivation in accordance with the present invention. MOTFT  10 ′ is similar to MOTFT  10  with similar components designated with similar numbers and a prime (′) is added to indicate a different structure. MOTFT  10 ′ does not include etch-stop/passivation area  20  and source/drain terminals  18 ′ are formed by etching directly a blanket layer of conductive metal. In MOTFT  10 ′ the conduction channel (defined by the space between the source/drain terminals  18 ′), is exposed to the ambient. A passivation insulator film  22 ′ is deposited over source/drain terminals  18 ′ and the conduction channel of MOTFT  10 ′. Passivation insulator film  22 ′ is one of the more porous insulators such as an organic polymer film, a PECVD SiO 2  film, or any of the insulating materials described above. MOTFT  10 ′ is then annealed back into oxygen vacancy equilibrium. With insulator film  22 ′ in place and metal oxide active layer  16 ′ in oxygen vacancy equilibrium, a dense metal film  24 ′ (similar to film  24  above) is deposited on insulator film  22 ′ so as to cover and protect the space between source/drain terminals  18 ′, which defines the conduction channel for MOTFT  10 ′. Thus, a composite film (polymer/metal or insulator/metal stack) is provided that is a good passivation film for MOTFT  10 ′. 
     Thus, the present invention provides a new and improved process for passivation of MOTFTs that improves environmental stability. Further, the present invention provides a new and improved process and material for passivation of MOTFTs that is compatible with flat panel technology. By depositing a more porous insulating film, annealing in oxidizing ambience to establish oxygen vacancy equilibrium, and depositing a dense inert/refractory metal and/or metal oxide layer a composite film (polymer/metal or insulator/metal stack) is provided that is a good passivation film for MOTFTs. Deposition of the dense metal layer is performed at low temperature which will not destroy the MOTFT. The dense metal is insulated from direct contact with the metal oxide (and, incidentally, the source/drain contacts) by the insulating film and insulated from the MOTFT. The composite film serves as a good barrier for external ambience, such as hydrogen, oxygen and water vapor, at moderate temperature (&gt;200° C., &lt;250° C.). On the other hand, oxygen from the metal oxide tends to diffuse through the insulating layer at elevated temperature. The dense metal layer serves as a barrier for the internal oxygen outflow. The oxidation of the inert metal at moderate temperature is slow enough that the threshold voltage of the MOTFT is not greatly changed. 
     The improved passivating structure (composite film) enables further processing at moderate temperature after MOTFT fabrication. Moderate temperature (200° C. to 250° C.) processes are needed in In-Plane-Switch LCD for PECVD SiON or SiN deposition. For X-ray imagers, there is a need to deposit a-Si photodiodes by PECVD at around 200° C. The improved passivating structure also allows the fabrication of a-Si photodiodes after the MOTFT back-plane is finished without destroying the MOTFTs in the photodiode fabrication process or the SiN passivation process for the photodiodes. 
     Various changes and modifications to the embodiment herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.