Abstract:
A ferroelectric film comprising polyaminodifluoroborane (PADFB). Also a memory device utilizing the ferroelectric film, a method of fabricating a ferroelectric polymer and a ferroelectric solution.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority to Singapore Priority Patent Application 200804448-9 filed in the Singapore Patent Office on May 30, 2008, the entire content of which is hereby incorporated by reference. 
     BACKGROUND 
     The present application relates to a ferroelectric polymeric film and a method of fabricating a ferroelectric polymer. 
     Ferroelectric polymers are promising materials for electronic devices. For example, they are suitable for a non-volatile memory. Since a thin film can be easily formed from a solution by means of a simple process such as spin-coating, die-coating, screen-printing or the other coating processes, the production cost of the devices utilizing a ferroelectric polymer is expected to be extremely cheap. However, remanence, which is the maximum spontaneous polarization when no external electric field is applied, of an organic material is generally lower than that of an inorganic ferroelectric material. Remanence is one of most important properties for a ferroelectric material. When a ferroelectric material is used for a memory device, the magnitude of the output signal is proportional to its remanence. So there exists a strong demand of a ferroelectric material which has an improved remanence. 
     SUMMARY 
     In an embodiment a ferroelectric polymer comprising Polyaminodifluoroborane (PADFB) is provided. This may have the advantage of increased remanence. 
     In another embodiment, a ferroelectric film comprising PADFB is provided. 
     In a further embodiment, a method of fabricating a ferroelectric polymer is provided including mixing a predetermined amount of PADFB with a solvent. 
     In yet another embodiment, a ferroelectric solution is provided including PADFB and a solvent. 
     Additional features and advantages of the present application are described in, and will be apparent from, the following Detailed Description and the figures. 
     Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a molecular structure of PVDF. 
         FIG. 2  is a molecular structure of PADBF. 
         FIG. 3  is the relationship between remanence and concentration of PADFB. 
         FIG. 4(   a ) is a device structure of 1T1C-type memory. 
         FIG. 4(   b ) is a device structure of 1T-type memory. 
         FIG. 4(   c ) is a device structure of cross point type memory. 
     
    
    
     DETAILED DESCRIPTION 
     The present application will be described below in greater detail with reference to the drawings according to an embodiment. 
     An example of a ferroelectric polymer is shown in  FIG. 1 . The polyvinylidenefluoride (PVDF) molecule  100  includes H—C bonds  102  and F—C bonds  104 . Ferroelectric properties of polymers may be related to the dipole moment induced between the hydrogen  106  and fluorine  108  atoms. This is also related to the polarity of the molecule. 
     A way to measure the dipole moment is by comparing the electronegativeness of the atoms within the molecule. The electronegativeness relates to the ability of an atom to attract electrons (or electron density) towards itself in a covalent bond. Since the electronegativeness of fluorine atom is 4.0 and that of carbon is 2.5, the fluorine atom tends to be charged more negatively than the carbon atom in a F—C bond. On the other hand, hydrogen atom has only 2.2 in electronegativeness. Therefore, the carbon atom tends to be charged more negatively than the hydrogen atom in an H—C bond. These properties induce imbalance of electron distribution, which leads to a strong dipole moment between the hydrogen and the fluorine atoms. 
     A further example of a ferroelectric polymer is shown in  FIG. 2 . The polyaminodifluoroborane (PADFB) molecule  200  includes B—F bonds  202  and N—H bonds  204 . The electronegativeness of boron  206  is 2.0 and nitrogen  208  is 3.0, and therefore PADFB may have a dipole moment twice as large as PVDF due to the large differences of electronegativeness in both B—F  202  and N—H  204  bonds. A larger dipole moment may have the advantage of higher remanence, since remanence is proportional to the maximum dipole moment obtained when no external electric field is applied. 
     PADFB may be synthesised by pyrolysis of precursor (BF 3 NH 3 ) and collected by a cold trap. Collected PADFB may be dissolved in dimethylformamide (DMF) and spin-coated onto a glass substrate. 
     Since PADFB is a hygroscopic and brittle material, it may be difficult to obtain a continuous and air-stable film. It may be desirable to provide a ferroelectric polymer with improved phase separation, low hygroscopicity and/or improved film stability in air while still obtaining high remanence. 
     Certain ferroelectric polymers have improved film uniformity and stability. For example ferroelectric polymers such as P(VDF/TrFE), PVDF or Nylon11 may be more air stable and less hygroscopic than PADFB. According to an embodiment, PADFB may be mixed with another material, for example a different ferroelectric polymer, to improve film uniformity and stability. 
       FIG. 3  shows the remanence for a range of mixtures of P(VDF/TrFE) and PADFB. This shows that remanence increases when the concentration of PADFB is higher, since PADFB has higher remanence compared to P(VDF/TrFE). As shown in  FIG. 3 , remanence tends to saturate between 10% and 20% in concentration of PADBF. 
     At least up until a concentration of 20%, most of PADFB molecules are surrounded by air-stable P(VDF/TrFE) molecules. Within this range the film was quite stable in air and significant absorption of water wasn&#39;t observed. A similar effect was observed when PADFB was mixed with various ferroelectric polymers such as PVDF or Nylon11. 
     A mixture of PADFB and P(VDF/TrFE) may be produced by dissolving 8 wt % of Dehydrated P(VDF/TrFE) pellets in dehydrated DMF in a glove box filled with nitrogen gas, then dissolving 0.8 wt % of PADFB, in the solution. The solution may be an intermediate product. The solution can then be spin-coated on a glass substrate in the glove box to avoid absorption of water and annealed at 150° C. for 3 hours. Alternatively for mass production a roll-to-roll process could be used where fabrication takes place on a running sheet-like substrate between two rolls of the sheet. For example, a 10% PADFB concentration film produced as described above exhibits good remanence. 
     Using a high remanence ferroelectric polymer in memory devices may allow the device to be smaller or more information can be stored in the same area.  FIGS. 4(   a )- 4 ( c ) illustrate various structures of memory devices. The structure shown in  FIG. 4(   a ) is called 1T1C type which consists of a transistor  400  and a capacitor  402  having a ferroelectric polymer according to the exemplary embodiment as the dielectric. To write information in this memory cell, an electric field higher than the coerciveness of the ferroelectric material is applied between BL and PL while the transistor  400  is turned on by applying voltage to WL for example 1.5 V which is sufficiently larger than the threshold level of the transistor. Depending on the polarity of the electric field between BL and PL, either data “0” or data “1” is stored in the ferroelectric layer. In a reading operation, a certain polarity of electric field is applied between BL and PL while the appropriate voltage is applied to WL. If the stored polarization reverses, a displacement current flows between BL and PL. On the other hand, if no polarization reversal is induced, no displacement current flows between BL and PL. Therefore, by detecting the displacement current, the stored information can be determined as having “0” or “1”. 
       FIG. 4(   b ) is a schematic of 1T type memory cell. The writing operation for the 1T type memory cell is the same as for the 1T1C type. An electric field higher than the coerciveness of the ferroelectric material  404  according to an embodiment is applied between BL and PL while the transistor  406  is turned on by applying an appropriate voltage to WL. To read the stored information in the cell, voltage is applied to WL for example 1.5 V while a certain magnitude of electric field smaller than the coerciveness is applied between BL and PL. Since the current flow between BL and PL differs depending on the direction of the stored polarization, the stored information can be determined as having “0” or “1”. 
       FIG. 4(   c ) shows a structure of so-called cross point type memory. It includes top electrodes  408 , bottom electrodes  410  and a ferroelectric layer  412  according to an embodiment. Top  408  and bottom  410  electrodes form a matrix and the ferroelectric layer  412  is sandwiched by these electrodes. When a pair of top  408  and bottom  410  electrodes is selected and a voltage is applied between them for example 30 V, an electric field exceeding coerciveness is induced at the corresponding cross point. Thus, writing or reading operation is executed following the same principle as 1T1C. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.