Patent Publication Number: US-2006003485-A1

Title: Devices and methods of making the same

Description:
BACKGROUND  
      Electronic devices, such as integrated circuits, may include thin film transistors (TFT). A TFT generally includes a gate electrode, a gate dielectric, a drain electrode, a source electrode, and a thin film semiconductor (channel) layer.  
      Gate dielectrics may generally be formed by deposition or growth processes that involve high-temperature processing (either during deposition/growth or as a post-processing step) to achieve acceptable performance. Some types of dielectric materials that can be processed at relatively low temperatures may have reduced long-term stability or reliability. Further, some dielectric materials may impose an upper temperature limit on downstream thermal processing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Objects, features and advantages will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though not necessarily identical components. For the sake of brevity, reference numerals having a previously described function may not necessarily be described in connection with subsequent drawings in which they appear.  
       FIG. 1  is a process flow diagram of embodiments of a method of forming an embodiment of a device;  
       FIG. 2  is an enlarged cross-sectional view of an embodiment of the device;  
       FIG. 3  is an enlarged cross-sectional view of an embodiment of the device having a tantalum layer; device; and  
       FIG. 5  is an enlarged cross-sectional view of an alternate embodiment of the device. 
    
    
     DETAILED DESCRIPTION  
      Embodiments of the disclosed method disclose processes for forming substantially transparent devices that may be used in circuits, including, but not limited to substantially transparent transistors and substantially transparent capacitors. The methods disclosed herein may be used in manufacturing processes, including, for example, integrating electrical circuits using mechanically flexible (e.g. plastic) substrates. One embodiment of the method includes forming a dielectric/gate dielectric via substantially complete anodization of a metal. This process may result in substantially transparent dielectrics/gate dielectrics with desired electrical properties. As referred to herein, substantially transparent, with reference to a structure, refers to transparency sufficient so that not less than about 50% of visible light energy incident on the structure is transmitted through the structure.  
      Referring now to  FIG. 1 , an embodiment of the method of making an embodiment of the substantially transparent device (non-limitative examples of which include substantially transparent transistors and capacitors) generally includes establishing a substantially transparent conductive layer  100 , establishing at least one metal layer  112 , forming a substantially transparent dielectric/gate dielectric from the metal layer by either substantially complete anodization  114  or substantially complete thermal oxidation  116 , and establishing a substantially transparent source, a substantially transparent drain, a substantially transparent channel, and/or a substantially transparent capacitor electrode  118 .  
      The various embodiments of the method form various embodiments of the substantially transparent devices.  FIGS. 2 through 4  are non-limitative representations of some of these embodiments. It is to be understood that different embodiments of the method may result in substantially transparent devices having substantially similar or different configurations.  
      Referring now to  FIG. 2 , in an embodiment of the method for making a substantially transparent device  10  (e.g. a substantially transparent (thin film) transistor or a substantially transparent capacitor), a substantially transparent conductive layer  12  is established on a substantially transparent substrate  14 . The substantially transparent conductive layer  12  may form a substantially transparent gate electrode  12 ′ (for a transistor) or a substantially transparent electrode  12 ′ (for a capacitor), depending on which device  10  is being fabricated. It is to be understood that any suitable material may be used for the substantially transparent conductive layer  12 . In an embodiment, this material is a doped transparent semiconductor material. One non-limitative example of a suitable transparent semiconductor material is indium tin oxide (ITO). Other examples of suitable doped semiconductor materials include, but are not limited to n-type doped indium oxide, n-type doped zinc oxide, n-type doped tin oxide, and/or mixtures thereof.  
      Further, it is to be understood that any suitable material may be used for the substantially transparent substrate  14 . Examples of suitable substantially transparent substrate  14  materials include, but are not limited to quartz, sapphire, glass, polycarbonates (PC), polyarylates (a non-limitative example of which is commercially available under the tradename ARYLITE from Promerus located in Brecksville, Ohio), polyethylene terephthalate (PET), polyestersulfones, polyimides (a non-limitative example of which is commercially available under the tradename KAPTON from DuPont located in Circleville, Ohio), polyolefins, polyethylene naphthalate (PEN), polyethersulfone (PES), polynorbornene (a non-limitative example of which is commercially available under the tradename APPEAR  3000  from Promerus located in Brecksville, Ohio), polyetheretherketone (PEEK), polyetherimide (PEI), and/or mixtures thereof.  
      The method further includes establishing one or more metal layer(s)  16  on the substantially transparent electrode/gate electrode  12 ′. It is to be understood that the metal selected for the one or more metal layer(s)  16  is dependent upon, among other factors, which embodiment of the method is being used to form the substantially transparent device  10 .  
      The method further includes forming a substantially transparent dielectric/gate dielectric  16 ′. This may be accomplished by either substantially complete anodization of the metal layer(s)  16  or substantially complete thermal oxidation of the metal layer(s)  16 . As referred to herein, substantially complete anodization or substantially complete oxidation refers to anodization or oxidation, respectively, performed to an extent such that the optical characteristics (for visible light) of device  10  are not significantly changed by further anodization or oxidation.  
      In an embodiment of the method, the established metal layer(s)  16  is substantially completely anodized throughout to form the substantially transparent dielectric/gate dielectric  16 ′. In this embodiment, the metal layer(s)  16  includes aluminum, tantalum, alloys thereof, and/or mixtures thereof. In an alternate embodiment, the metal layer(s)  16  includes one or more aluminum layer(s) and one or more tantalum layer(s). Other suitable metals for the anodization method may include, but are not limited to, bismuth, antimony, niobium, silver, cadmium, iron, magnesium, tin, tungsten, zinc, zirconium, titanium, copper, chromium, alloys thereof, and/or mixtures thereof. The thickness of the metal layer(s)  16  ranges between about 10 nm and about 500 nm. It is to be understood that the substantially complete anodization process forms an oxide of the selected metal. Thus, in a non-limitative embodiment(s), the formed substantially transparent dielectric/gate dielectric  16 ′ is aluminum oxide (alumina) and/or tantalum pentoxide.  
      In an embodiment, the substantially complete anodization of aluminum and/or tantalum may take place at room temperature, and/or, more generally, at any temperature above the freezing temperature and below the boiling temperature of the selected electrolyte. In a non-limitative example, aluminum is substantially completely anodized through using a citric acid electrolyte (C 6 H 8 O 7  or HOCOH 2 C(OH)(COOH)CH 2 COOH, 1 wt. % in water), an aluminum cathode (99.99% purity), and about 5 mA/cm 2  current density to achieve the desired and/or suitable voltage (anodization coefficient for anodic alumina in citric acid is ˜1.3 nm of alumina per 1 volt). Other non-limitative examples of suitable electrolytes include those based on boric acid (H 3 BO 3 ), ammonium pentaborate ((NH 4 ) 2 B 10 O 16 ), ammonium tartrate (H 4 NO 2 CCH(OH)CH(OH)CO 2 NH 4 ), and the like. In a non-limitative example, tantalum is substantially completely anodized using a platinum or stainless steel cathode and a boric acid electrolyte with pH adjusted to about 7 by ammonia, and a current density of about 0.05 mA/cm 2  to achieve the desired and/or suitable voltage and, as a result, thickness (anodization coefficient for anodic tantalum pentoxide is −1.8 nm of tantalum pentoxide per 1 volt).  
      It is to be understood that a dual anodization process may also optionally be used, for example, when oxidizing more than ˜350 nm of metal. This generally includes the fabrication of porous anodic alumina (oxalic acid, sulfuric acid, phosphoric acid, and/or mixtures thereof as electrolytes) and the fabrication of a barrier type of anodic alumina (non-limitative examples of which include citric acid, boric acid, ammonium pentaborate, and ammonium tartrate as electrolytes). Suitable solvents for this process include, but are not limited to water, alcohols, and/or mixtures thereof. It is to be understood that organic solvents may also be added to the solvent used. It is to be understood that for barrier type anodic alumina and tantalum pentoxide, anodized film thickness is a function of the anodization voltage (˜1.3 nm per volt for alumina and ˜1.8 nm per volt for tantalum pentoxide), while for porous oxides, the thickness is proportional to the cumulative charge density (i.e., film thickness is proportional to the product of anodization current density and the time for which this current flows, or the integrated anodization current density with respect to time).  
      In an alternate embodiment of the method, the metal layer(s)  16  is substantially completely thermally oxidized in air to form the substantially transparent dielectric/gate dielectric  16 ′. It is to be understood that nitrogen may also be a suitable atmosphere for nitridation [M+N 2 —&gt;M x N y  or nitride], depending on the metal being oxidized. In this embodiment, the metal layer(s)  16  is tantalum and has a thickness ranging between about 10 nm and about 500 nm. The temperature of the substantially complete thermal oxidation ranges between about 300° C. and about 600° C. It is to be understood that a predetermined amount of tantalum is established for the metal layer(s)  16  and corresponds to a predetermined temperature such that a desired and/or suitable amount of tantalum pentoxide (the substantially transparent dielectric/gate dielectric  16 ′) is formed.  
      The combination of the substantially transparent dielectric/gate dielectric  16 ′ and the substantially transparent electrode/gate electrode  12 ′ forms a substantially transparent stack/gate stack  18  disposed on the substantially transparent substrate  14 . It is to be understood that the substantially transparent stack/gate stack  18  may be subject to further processing steps (including the establishment of additional layers on the stack/gate stack  18  and/or between the layers of the stack/gate stack  18 ) and may ultimately be operatively disposed in the substantially transparent device  10 .  
      Whether the metal layer(s)  16  is substantially completely anodized or substantially completely thermally oxidized, the method may further include establishing a substantially transparent source  20 , a substantially transparent drain  22 , a substantially transparent channel  24 , and/or a substantially transparent capacitor electrode  26  (as shown in  FIG. 5 ) on the substantially transparent dielectric/gate dielectric  16 ′. It is to be understood that these substantially transparent elements  20 ,  22 ,  24  and  26  may be composed of any suitable materials, including, but not limited to substantially transparent semiconductor materials. Suitable non-limitative examples of these materials for a channel layer  24  include zinc oxide, tin oxide, cadmium oxide, indium oxide, n-type doped zinc oxide, n-type doped tin oxide, n-type doped cadmium oxide, n-type doped indium oxide, and/or mixtures thereof. Suitable non-limitative examples of these materials for source  20 , drain  22 , and capacitor electrode  26  include n-type doped zinc oxide, n-type doped tin oxide, n-type doped cadmium oxide, n-type doped indium oxide, and/or mixtures thereof.  
      As shown in the Figures, it is to be further understood that the source  20  and drain  22  may be interchangeable, i.e. if source  20  is on the left, drain  22  will be on the right; and if drain  22  is on the left, source  20  will be on the right.  
      It is to be understood that in an embodiment using substantially complete thermal oxidation, the substantially transparent source  20 , drain  22 , channel  24 , and/or capacitor electrode  26  may be established either before or after the thermal oxidation of the metal (tantalum) layer(s)  16  in order to form the embodiment of the substantially transparent device  10  shown in  FIG. 2 .  
      Any suitable establishment (deposition) method may be used to deposit the substantially transparent conductive material/layer  12 , the metal layer(s)  16 , and the substantially transparent source  20 , substantially transparent drain  22 , substantially transparent channel  24 , and the substantially transparent capacitor electrode, if employed. In an embodiment, establishing is accomplished by at least one of sputtering, chemical vapor deposition (CVD), atomic layer deposition (ALD), evaporation (e.g. thermal or e-beam), inkjet deposition, and/or spin-coating.  
      As described hereinabove, the substantially transparent device  10  illustrated in  FIG. 2  may be formed by an embodiment of the method incorporating substantially complete anodization of the established metal layer(s)  16  or an embodiment of the method incorporating substantially complete thermal oxidation of the metal (tantalum) layer(s)  16  (either before or after the establishment of the substantially transparent source  20 , drain  22 , channel  24 , and/or capacitor electrode  26 ).  
      Referring now to  FIG. 3 , an embodiment of the method may optionally include establishing a layer  28  on the substantially transparent electrode/gate electrode  12 ′, prior to the establishment of the metal layer(s)  16 . It is to be understood that this layer  28  may be disposed between the substantially transparent electrode/gate electrode  12 ′ and the substantially transparent dielectric/gate dielectric  16 ′ in the resulting substantially transparent device  10 . The layer  28  includes tantalum, tantalum oxides, and/or mixtures thereof. In an embodiment, the thickness of the layer  28  ranges between about 1 nm and about 50 nm. One non-limitative embodiment includes a layer  28  having a thickness ranging between about 1 nm and about 10 nm. A non-limitative example of the layer  28  is tantalum.  
      Without being bound to any theory, it is believed that the addition of the layer  28  may advantageously aid in the substantially complete anodization of the metal layer(s)  16 . The layer  28  may act as a conductor, thereby aiding in substantially fully and uniformly anodizing the metal layer(s)  16 . It is further believed that the layer  28  may, in some instances, substantially prevent the break-down of the anodic alumina film, achieve an increase in the adhesion of the metal layer(s)  16 , and/or may provide a substantially uniform electrical field distribution at the final stages of anodization.  
       FIG. 4  illustrates an alternate embodiment of the substantially transparent device  10 . It is to be understood that the materials and establishment (deposition) techniques as previously described may be employed in this embodiment of the method.  
      The method includes first establishing the substantially transparent source  20 , drain  22 , the channel  24 , and/or the capacitor electrode  26  on the substantially transparent substrate  14 .  
      The metal layer(s)  16  is then established on the substantially transparent source  20 , drain  22 , the channel  24 , and/or the capacitor electrode  26  and on any exposed portion of the substantially transparent substrate  14 . In this embodiment, the metal layer(s)  16  is tantalum.  
      The substantially transparent conductive layer  12  is established on the metal layer(s)  16 , thereby forming the substantially transparent electrode/gate electrode  12 ′. As depicted in  FIG. 4 , this embodiment of the substantially transparent device  10  has the substantially transparent electrode/gate electrode  12 ′ formed over the substantially transparent dielectric/gate dielectric  16 ′ as opposed to an embodiment where the substantially transparent dielectric/gate dielectric  16 ′ is formed over the substantially transparent electrode/gate electrode  12 ′ (see  FIGS. 2 and 3 ).  
      The method further includes substantially completely thermally oxidizing the metal layer(s)  16  to form the substantially transparent dielectric/gate dielectric  16 ′. It is to be understood that the thermal oxidation process forms an oxide of the tantalum metal. Thus, in this embodiment, the formed substantially transparent dielectric/gate dielectric  16 ′ is tantalum pentoxide.  
      Embodiments of the device  10  include a substantially transparent substrate  14 , a substantially transparent electrode  12 ′ or a substantially transparent gate electrode  12 ′, a substantially transparent dielectric or a substantially transparent gate dielectric  16 ′ (formed by either substantially complete anodization or thermal oxidation), and a substantially transparent source  20 , drain  22 , channel  24  and/or capacitor electrode  26 . It is to be understood that the device  10  may be any suitable device, including, but not limited to substantially transparent thin film transistors and substantially transparent capacitors.  
       FIG. 5  shows a capacitor as the device  10 , with a substantially transparent capacitor electrode  26  operatively disposed on the substantially transparent dielectric  16 ′.  
      A method of using an embodiment of the substantially transparent gate stack  18  disposed on a substantially transparent substrate  14  includes establishing the substantially transparent source  20  and the substantially transparent drain  22  on the substantially transparent gate stack  18 . The method further includes operatively disposing the substantially transparent gate stack  18  having the source  20  and drain  22  disposed thereon in a device  10 .  
      Embodiments of the devices  10  and methods of forming the same according to embodiments disclosed herein may be used for forming substantially transparent devices  10 , including, but not limited to transistors and capacitors. The methods disclosed herein may be used in manufacturing processes, including, for example, integrating electrical circuits using mechanically flexible (e.g. plastic) substrates. Forming a substantially transparent dielectric/gate dielectric  16 ′ via substantially complete anodization of a metal layer  16  may result in substantially transparent dielectric/gate dielectrics  16 ′ having desirable electrical properties.  
      While embodiments have been described in detail, it will be apparent to those skilled in the art that the disclosed embodiments may be modified. Therefore, the foregoing description is to be considered exemplary rather than limiting.