Abstract:
Bismuth lanthanum titanate (BLT) having a formula Bi 4−x La x Ti 3 O 12 , a method for preparing the BLT, a BLT thin film formed of the BLT, a method for forming the BLT thin film, and an electronic device including the BLT thin film, where 0&lt;x≦2.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to bismuth lanthanum titanate (BLT), a BLT thin film, and an electronic device employing the BLT film, and more particularly, to BLT having a ferroelectric characteristic, a BLT thin film, and an electronic device whose fatigue characteristic is improved by adopting the BLT thin film.  
           [0003]    2. Description of the Related Art  
           [0004]    With the advance of portable information communications device, the demand for non-volatile memory device capable of retaining information stored therein even when power is turned off is increasing. A ferroelectric RAM (FeRAM) as an electronic device which satisfies the demand has advantages in terms of rapid information writing speed and low power consumption.  
           [0005]    In the manufacture of the FeRAM, a capacitor is manufactured by forming upper and lower electrodes on both sides of a ferroelectric thin film. A PZT(PbZr X Ti 1−X O 3 ) (where 0≦X&lt;1) film or SBT(SrBi 2 Ta 2 O 9 ) film is used as the ferroelectric thin film. The fatigue characteristic of the FeRAM is poor when the ferroelectric thin film is formed of the PZT film. To solve this problem, as the lower electrode layer, both a metal electrode formed of platinum (Pr) or iridium (Ir), and a metal oxide electrode formed of a metal oxide such as IrO 2  or RuO 2  are used.  
           [0006]    However, the metal oxide electrode is formed at high temperature and high cost. Due to this processing problem, the metal oxide electrode cannot be manufactured on mass scale.  
           [0007]    On the other hand, when the SBT film is used as the ferroelectric thin film, the fatigue characteristic is good. However, a desired ferroelectric characteristic is ensured only when formation of the thin film is performed at high temperature. Also, in manufacturing a highly integrated memory device by a poly-plug process, polysilicon of a connect portion between an active region of the memory and a lower electrode on which the ferrorelectric film is to be grown is oxidized as the SBT thin film is crystallized at high temperature. As a result, due to the problem of contact resistance, the electronic device operates incorrectly.  
         SUMMARY OF THE INVENTION  
         [0008]    To solve the above-described problems, it is a feature of the present invention to provide bismuth lanthanum titanate (BLT) which can be used to form a ferroelectric thin film having a low crystallization temperature and improved fatigue characteristic, a BLT thin film, and methods for preparing the BLT and forming the BLT thin film.  
           [0009]    It is another feature of the present invention to provide an electronic device whose fatigue characteristic is improved by adopting the BLT thin film.  
           [0010]    In accordance with one aspect of the present invention, there is provided a bismuth lanthanum titanate (BLT) having formula (1): 
           Bi 4−x La x Ti 3 O 12   (1) 
           [0011]    where 0&lt;x≦2.  
           [0012]    In accordance with another aspect of the present invention, there is provided a method for preparing a bismuth lanthanum titanate (BLT) having formula (1): 
           Bi 4−x La x Ti 3 O 12   (1) 
           [0013]    where 0&lt;x≦2, the method comprising the steps of: dissolving a bismuth-containing salt in a first solvent and a second solvent to obtain a bismuth solution; dissolving a lanthanum-containing salt in a third solvent to obtain a lanthanum solution, mixing the lanthanum solution and the bismuth solution to obtain a bismuth lanthanum solution, adding a titanium-containing compound to the bismuth lanthanum solution, and mixing the mixture to obtain a bismuth lanthanum titanium solution; and drying the resultant product of the previous step to remove the solvents, and thermally processing the dried product.  
           [0014]    In formula (1), it is preferable that x is in the range of about 0.7-0.75.  
           [0015]    In accordance with a further aspect of the present invention, there is provided a bismuth lanthanum titanate (BLT) thin film formed of BLT having formula (1): 
           Bi 4−x La x Ti 3 O 12   (1) 
           [0016]    where 0&lt;x≦2. The BLT thin film can be formed by coating a composition for the BLT having formula (1) above on a substrate, and drying and thermally processing the resultant structure. The composition can be prepared by mixing a bismuth solution obtained by dissolving a bismuth-containing salt in a first solvent and a second solvent, a lanthanum solution obtained by dissolving a lanthanum-containing salt in a third solvent, and a titanium-containing compound.  
           [0017]    The present invention also provides a method for forming a bismuth lanthanum titanate (BLT) thin film having formula (1): 
           Bi 4−x La x Ti 3 O 12   (1) 
           [0018]    where 0&lt;x≦2, the method comprising the steps of: dissolving a bismuth-containing salt in a first solvent and a second solvent to obtain a bismuth solution; dissolving a lanthanum-containing salt in a third solvent to obtain a lanthanum solution, mixing the lanthanum solution and the bismuth solution to obtain a bismuth lanthanum solution, adding a titanium-containing compound to the bismuth lanthanum solution, and mixing the mixture to obtain a bismuth lanthanum titanium solution; and coating the substrate with the bismuth lanthanum titanium solution obtained in the preceding step, and drying and thermally processing the coated product.  
           [0019]    In accordance with still another aspect of the present invention, there is provided an electronic device including a BLT thin film as described above. A capacitor can be formed by depositing a conductive layer on and underneath the BLT thin film. In this case, as the conductive layer, a metal electrode formed of at least one selected from the group consisting of Pt, Ru and Ir, or a metal oxide electrode formed of at least one metal oxide selected from the group consisting of IrO 2  and RuO 2 , can be used. Although one of the metal electrode and metal oxide electrode is used as the conductive layer, the capacitor has good fatigue characteristic.  
           [0020]    In particular embodiments, the electronic device can be a ferroelectric memory device or a pyroelectric sensor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The above features and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0022]    [0022]FIG. 1 is a schematic view of the structure of a spin coating apparatus for use in the deposition of a bismuth lanthanum titanate (BLT) thin film according to the present invention;  
         [0023]    [0023]FIG. 2 shows the X-ray diffraction pattern of BLT thin films formed in Examples 1 through 3 according to the present invention;  
         [0024]    [0024]FIGS. 3A and 3B are scanning electron microscope (SEM) images (50,000×) of BLT thin films formed in Examples 2 and 3 according to the present invention;  
         [0025]    [0025]FIGS. 4A and 4B show the P-E characteristic of BLT thin films formed in Examples 2 and 3 according to the present invention;  
         [0026]    [0026]FIG. 5A shows the fatigue characteristic of a PZT thin film;  
         [0027]    [0027]FIG. 5B shows the fatigue characteristic of the BLT thin film formed in Example 3 according to the present invention;  
         [0028]    [0028]FIGS. 6A through 6E are sectional views illustrating a method for manufacturing a preferred embodiment of a memory device according to the present invention; and  
         [0029]    [0029]FIGS. 7A through 7C are sectional views of another embodiment of the memory device adopting a BLT thin film according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Korean Patent Application No. 00-30796, filed Jun. 5, 2000, and entitled: “LBT Solution, Method for Fabricating LBT Solution and Method for Fabricating LBT Thin Film and Electric Device Using the Same,” is incorporated by reference herein in its entirety.  
         [0031]    Bismuth lanthanum titanate (BLT) according to the present invention as a ferroelectric substance is expressed as formula (1), which is obtained by partially substituting bismuth (Bi) atoms of bismuth titanate (Bi 4 Ti 3 O 12 ) with lanthanum (La) atoms: 
         Bi 4−x La x Ti 3 O 12   (1) 
         [0032]    where 0&lt;x≦2.  
         [0033]    In formula (1), it is preferable that x is in the range from about 0.7 to about 0.75, and more preferably, about 0.75. This is because the Curie temperature of the ferroelectric BLT at x=0.75 is about 400° C., so that the residual polarization value is very large at 30 μC/cm 2  (C-axial orientation).  
         [0034]    A method for preparing the BLT having formula (1) above will now be described. First, a bismuth containing salt is dissolved in a first solvent and then in a second solvent to obtain a bismuth solution. It is preferable that the bismuth solution is prepared at a temperature of about 60-85° C., and more preferably, about 60-75° C. This is because a uniform bismuth solution can be attained in the range of temperature described above.  
         [0035]    Any bismuth-containing salt can be used. Preferably, bismuth acetate, bismuth nitrate, or bismuth acetylacetonate is used. As the first solvent for use in dissolving the bismuth-containing salt, an amine compound can be used. For example, suitable amine compounds include pyridine, triethylamine, trimethylamine, and polyamine. The second solvent, serving as a co-solvent, preferably includes acetic acid, 2-methoxy ethanol, n-propanol, and 2-ethylhexanoic acid. Here, the first and second solvents are mixed in a weight ratio of about 1:15-1:5, and preferably, about 2:15. It is preferable that the total amount of the first and second solvents is in the range of about 2500-2800 parts by weight based on 100 parts by weight of the bismuth-containing salt.  
         [0036]    Following this, a lanthanum-containing salt is dissolved in a third solvent and mixed with the bismuth solution to obtain a bismuth lanthanum solution. Any lanthanum-containing salt can be used. Preferably, lanthanum acetate or lanthanum nitrate is used as the lanthanum-containing salt. As the third solvent for use in dissolving the lanthanum-containing salt, acetic acid, 2-methoxyethanol, n-propanol, or 2-ethylhexanoic acid is preferred. It is preferable that the amount of the third solvent is in the range of about 400-500 parts by weight based on 100 parts by weight lanthanum-containing salt. It is preferable that the lanthanum solution is prepared at a temperature of about 60-85° C., and more preferably, about 60-75° C. This is because a uniform lanthanum solution can be attained in the range of temperature described above.  
         [0037]    A titanium-containing compound is added into the bismuth lanthanum solution prepared through the above processes, and mixed to obtain a bismuth lanthanum titanium solution. Preferably, the bismuth lanthanum titanium solution is prepared at a temperature of about 60-85° C., and more preferably, about 60-75° C., as in the preparation of the bismuth and lanthanum solutions. The bismuth lanthanum titanium solution can be cooled down to room temperature and filtered to remove impurities, as needed. A higher purity bismuth lanthanum titanate (BLT) is advantageously obtained through the filtration.  
         [0038]    As the titanium containing compound, titanium(di-isopropoxide)bis (acetylacetonate) or titanium isopropoxide preferably is used.  
         [0039]    Following this, the resultant product is dried to remove the first, second, and third solvents, and a thermal process is performed to obtain BLT. It is preferable that the drying process is performed at a temperature of about 250-600° C. If the drying temperature is less than about 250° C., the solvent in the thin film cannot evaporate completely. It is preferable that the thermal process is performed at a temperature of about 600-750° C. If the temperature of the thermal process is less than about 600° C., the electric characteristic is poor. If the temperature of the thermal process is above about 750° C., it is undesirable for the manufacture of memory.  
         [0040]    In the preparation of BLT having formula (1) above, the lanthanum-containing salt preferably is used in an amount of about 0.75 mol based on 3.25 mol bismuth-containing salt. Preferably, the titanium-containing compound is used in an amount of about 3 moles.  
         [0041]    A method for forming a BLT thin film according to the present invention will be described with reference to FIG. 1. Referring to FIG. 1, a substrate  11  is mounted on a rotatable chuck  10  and the chuck  10  is rotated. It is preferable that the rotation speed of the chuck  10  is in the range of about 2000-4000 rpm. A composition for the BLT having formula (1) is applied to the top of the substrate  11  through a supply pipe  12  and spin-coated thereon.  
         [0042]    The composition BLT having formula (1) above is prepared, as described above, by mixing the bismuth solution obtained by dissolving a bismuth-containing salt in the first and second solvents, the lanthanum solution obtained by dissolving a lanthanum-containing salt in the second solvent, and a titanium-containing compound. The resultant structure is dried at a temperature of about 250-600° C. Here, the drying time is varied depending on the drying temperature. The drying process is performed within 10 minutes in the range of temperature described above. These procedures are repeatedly conducted to obtain a target film thickness.  
         [0043]    After the drying process described above is completed, the substrate is thermally processed at a temperature of about 600-750° C. to form a BLT thin film according to the present invention. Here, the thermal process may be performed by rapid thermal annealing (RTA) or by using a tube furnace in a separate thermal processing chamber equipped with a hot plate. When the thermal process is performed by the RTA, it is preferable that the RTA temperature is in the range of about 650-750° C. If the RTA temperature is less than about 650° C., the ferroelectric characteristic is poor. If the RTA temperature is above about 750° C., it is undesirable for the manufacturing process. When the thermal process is performed using a tube furnace, it is preferable that the thermal process is performed at a temperature of about 600-700° C. If the thermal process using the tube furnace is performed at less than about 600° C., the ferroelectric characteristic is poor. If the thermal process using the tube furnace is performed above about 700° C., it is undesirable for the manufacture of ferroelectric memory.  
         [0044]    A capacitor is manufactured by forming a conductive film on and underneath the BLT thin film formed by the above process. Here, as the conductive layer, a metal electrode formed of platinum (Pt), ruthenium (Ru), or iridium (Ir), or a metal oxide electrode formed of IrO 2  or RuO 2 , can be used. The metal electrode is preferred as the conductive film because it is easy to manufacture with a high reproducibility and has a better surface roughness characteristic compared to the metal oxide electrode.  
         [0045]    A variety of electronic devices, for example, a ferroelectric memory (FeRAM) or pyroelectric sensor, can be manufactured using the capacitor.  
         [0046]    The present invention will be described in greater detail by means of the following examples. The following examples are for illustrative purposes and are not intended to limit the scope of the invention.  
       EXAMPLE 1  
       [0047]    To prepare a 30 ml BLT solution, 3.80 g bismuth acetate was dissolved in 4 ml pyridine, to which 10 ml acetic acid was added. The solution was stirred at about 65° C. to obtain a bismuth solution. Next, 20 ml acetic acid was added to 0.711 g lanthanum acetate, and the mixture was stirred at about 65° C. to obtain a lanthanum solution. The bismuth solution and the lanthanum solution were mixed, 4.24 g titanium(di-isopropoxide)bis(acetylacetonate) was added to the mixture, and the mixture was stirred at 80° C. for about 1 hour. The mixture was cooled down to room temperature and filtered through a 0.2 μm-filter to remove particles from the BLT solution. The filtrate was obtained as a composition for BLT.  
         [0048]    The chuck  10  of FIG. 1 was rotated at about 2000 rpm and the composition for BLT was applied onto the surface of the silicon substrate  11  through the supply pipe  12 , so that the surface of the silicon substrate  11  was spin-coated with the composition. The resultant structure was dried at 300° C. for 3-5 minutes. The above procedures were conducted to obtain a target film thickness. The resultant structure was moved into a thermal processing chamber equipped with a hot plate to thermally process the resultant structure using a tube furnace at about 600° C. for 30 minutes, thereby resulting in a BLT thin film on the silicon substrate  11 .  
       EXAMPLE 2  
       [0049]    A BLT thin film was formed in the same manner as in Example 1, except that the thermal process was performed at a temperature of 650° C.  
       EXAMPLE 3  
       [0050]    A BLT thin film was formed in the same manner as in Example 1, except that the thermal process was performed at a temperature of 700° C.  
         [0051]    X-ray diffraction pattern analysis was performed for the BLT thin films formed in Examples 1 through 3. The result is shown in FIG. 2. As shown in FIG. 2, it is apparent that BLT crystals are formed in the range of temperature applied in the examples.  
         [0052]    Meanwhile, the cross-sections of the BLT thin films formed in Example 2 and 3 were observed. The results are shown in FIGS. 3A and 3B. FIGS. 3A and 3B are scanning electron microscope (SEM) images (50,000×) of the BLT thin films formed in Examples 2 and 3. For the BLT thin film processed at 650° C. (Example 2), as shown in FIG. 3A, crystal grains appearing as BLT crystals are shown. As shown in FIG. 3B, when the thermal process is performed at 700° C. (Example 3), a BLT thin film including more cavities is formed. In addition, as shown in FIGS. 3A and 3B, the BLT thin films have a thickness of about 0.23 μm and the surface thereof is fairly level. Thus, the BLT thin films according to the present invention are useful in forming a variety of electronic devices in terms of film thickness and surface characteristic.  
         [0053]    The P-E characteristic was evaluated for the BLT thin films of Examples 2 and 3. The results are shown in FIGS. 4A and 4B. Referring to FIGS. 4A and 4B, both the BLT films of Examples 2 and 3 have a ferroelectric characteristic. It is also apparent that the crystallization of BLT is easier when the thermal process is performed at 650° C. and 700° C. In particular, the residual polarization value (Pr=P*−P^ r) of BLT film is larger at 700° C. (FIG. 4B) than at 650° C. (FIG. 4A).  
         [0054]    In addition, the fatigue characteristic was measured for the BLT thin film of Example 3. For the fatigue characteristic measurement, a fatigue voltage of 3V at a frequency of 1 MHz was applied, and the amount of polarization of the BLT thin film at 5V was measured. The result of the fatigue characteristic measurement is shown in FIG. 5B.  
         [0055]    Referring to FIG. 5B, the variation in the amount of both switching (P sw ) and non-switching polarization (P ns ) is very small with increased number of measurement cycles. Also, the variation in the difference between these two values does not become greater with increased number of measurement cycles. When the number of the measurement cycles is greater than 2.5×10 10 , the variation in the amount of polarization is small at about less than 10%. Thus, it is evidenced that the fatigue characteristic of the BLT thin film is much better than that of a PZT thin film shown in FIG. 5A.  
         [0056]    [0056]FIGS. 6A through 6E are sectional views illustrating a preferred embodiment of a ferroelectric memory device fabricating method using a BLT thin film according to the present invention. FIG. 6A shows the step of forming a contact hole  26  in a substructure  22  formed on silicon substrate  20 . Referring to FIG. 6A, the contact hole  26  is formed by photolithography in the substructure  22  having constituent elements such as a bit line  24 .  
         [0057]    [0057]FIG. 6B shows the step of forming a node of a capacitor on the substructure  22  formed on the silicon substrate  20 . Referring to FIG. 6B, a polysilicon layer  28  is deposited on the silicon substrate  20  to fill the contact hole  26 . The polysilicon layer  28  is etched such that the polysilicon layer  28  remains only in the contact hole  26 , so that the node of the capacitor is completed.  
         [0058]    [0058]FIG. 6C shows the step of forming a lower electrode  30 . An iridium oxide film is deposited on the substructure  22  and the lower electrode  30  is formed by photolithography. Here, platinum (Pt) is formed as a material of the lower electrode  30 .  
         [0059]    [0059]FIG. 6D shows the step of forming a dielectric film on the lower electrode  30 . The silicon substrate  20  is mounted on the rotatable chuck  10 , as shown in FIG. 1. The composition for BLT is applied on the top of the silicon substrate through the supply pipe  12  while rotating the chuck  10 . The resultant structure is dried and thermally processed to form a BLT thin film  32 . In particular, the composition for BLT coated on the substructure  22  formed on silicon substrate  20  is dried at a temperature of 250-400° C. for 3-6 minutes, and the resultant silicon substrate is transferred into a thermal processing chamber equipped with a hot plate and processed by RTA, so that the BLT thin film  32  is formed.  
         [0060]    [0060]FIG. 6E shows the step of forming an upper electrode  34  on the BLT thin film  32 . A conductive material such as Pt is deposited on the BLT thin film  32  to form the upper electrode  34 , so that a capacitor  36  is completed.  
         [0061]    [0061]FIGS. 7A through 7C are sectional views illustrating the structure of other memory devices employing the BLT thin film according to the present invention. In particular, FIG. 7A shows the structure of a single transistor memory device using a BLT thin film according to the present invention. FIG. 7B shows the structure of a one-transistor and one-capacitor ( 1 Tr- 1 C) memory device using a BLT thin film according to the present invention. FIG. 7C shows the structure of a COB (Capacitor-On-Bit line) type  1 Tr- 1 C memory using a BLT thin film according to the present invention.  
         [0062]    In FIGS. 7A through 7C, reference numeral  40  represents a silicon substrate, reference numeral  41  represents an active region, reference numeral  42  represents a non-active region, reference numeral  43  represents a substructure, reference numeral  44  represents a gate, reference numeral  45  represents a polysilicon layer, reference numeral  46  represents a lower electrode, reference numeral  48  represents a BLT thin film, reference numeral  50  represents an upper electrode, and reference numeral  52  represents a capacitor. From the ferroelectric memory devices shown in FIGS. 7A through 7C, it is evidenced that a reliable electronic device which exhibits a good fatigue characteristic can be manufactured using only a Pt electrode in forming the lower electrode  46  without the need for an IrO 2  electrode.  
         [0063]    Although the embodiments of FIGS. 7A through 7C are illustrated with reference to a high-capacitance ferroelectric memory device, the present invention is applicable to any electronic device using a ferroelectric material.  
         [0064]    The ferroelectric BLT having formula (1) above according to the present invention can be processed into a thin film. Although only a metal oxide electrode or metal electrode is used alone as a conductive layer formed on and underneath the BLT thin film, a reliable high-capacitance capacitor which exhibits a good fatigue characteristic can be manufactured. This capacitor can be applied to form an electronic device such as a ferroelectric memory device or pyroelectric sensor.  
         [0065]    While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.