Abstract:
A semiconductor memory device includes a memory cell including a ferroelectric capacitor and a cell transistor and storing binary data at a first potential level and a second potential level which is higher than the first potential level, a bit line which reads the binary data from the memory cell, a correction circuit which corrects an error of the binary data read from the memory cell via the bit line, and a setting circuit which sets to the first potential a potential of the bit line connected to the memory cell from which at least the binary data is read, after the binary data is transferred to the correction circuit. The device further includes a control circuit which controls the potential of the bit line connected to the memory cell from which the binary data is read, in accordance with a result of error correction of the binary data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-269061, filed Sep. 15, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor memory device. More specifically, the invention relates to a ferroelectric random access memory (FeRAM) having an error checking and correcting (ECC) circuit and using a ferroelectric capacitor.  
         [0004]     2. Description of the Related Art  
         [0005]     Attention has recently been attracted to a ferroelectric random access memory using a ferroelectric capacitor as a nonvolatile semiconductor memory device. The ferroelectric random access memory nonvolatilely stores binary data according to the amount of different two polarizations (amount of remanent polarization) of a ferroelectric using the fact that spontaneous polarization, one of characteristics of the ferroelectric exhibits a hysteresis characteristic.  
         [0006]     The memory cells (unit cells) of a prior art FeRAM generally adopt the same architecture as those of a dynamic random access memory (DRAM). In the prior art FeRAM, a paraelectric capacitor of the DRAM is replaced with a ferroelectric capacitor and the ferroelectric capacitor is connected in series to a cell transistor to form a unit cell. Such unit cells are arranged two-dimensionally to configure a large-capacity memory cell array.  
         [0007]     There are two types of FeRAM. One is a so-called 2T/2C FeRAM from which data is read using two unit cells, and the other is a so-called 1T/1C FeRAM from which data is read using one unit cell.  
         [0008]     Data is read from an FeRAM by applying a voltage to a ferroelectric capacitor and reversing the polarization. In other words, the read of data from the FeRAM is destructive read. It is thus necessary to write back the read data. In general, the compared and amplified state of read data is maintained by a sense amplifier to write data “0.” Then, the potential of a plate line drops to VSS from the state to write data “1.” It is more effective to write data “0” first and then data “1” as a data write-back operation.  
         [0009]     In the above destructive-read type FeRAM, data is repeatedly written each time data is read out. Particularly in an FeRAM with an ECC circuit, read data is written back in accordance with the results of error correction and correct data can be written. The ECC circuit can thus improve the reliability of the FeRAM.  
         [0010]     However, it takes appropriate time to perform an error correcting operation in the ECC circuit. The following problem therefore occurs. When data is written back after its error is corrected, time required for writing back the data, including time required for correcting the error, is lengthened. In other words, when data is written back in accordance with the results of error correction in the ECC circuit, the results of error correction can be reflected in the data write-back operation, but cycle time is lengthened.  
         [0011]     When data “0” is written first and then data “1” is done as a data write-back operation, the error correction in the ECC circuit has only to be completed at least by the end of writing of data “0” in order to reflect the results of the error correction. In other words, if time required for the error correction is shorter than that required for writing data “0,” the results of the error correction can be reflected in the writing of data “1” without lengthening cycle time. If, however, time required for the error correction is longer than that required for writing data “0,” the cycle time is lengthened accordingly.  
         [0012]     As a technique related to the above, there is proposed a technique of improving the precision of detection of errors of data (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-16389).  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     According to an aspect of the present invention, there is provided a semiconductor memory device comprising a memory cell including a ferroelectric capacitor and a cell transistor and storing binary data at a first potential level and a second potential level which is higher than the first potential level, a bit line which reads the binary data out of the memory cell, a correction circuit which corrects an error of the binary data read out of the memory cell via the bit line, a setting circuit which sets to the first potential a potential of the bit line connected to the memory cell from which at least the binary data is read, after the binary data is transferred to the correction circuit, and a control circuit which controls the potential of the bit line connected to the memory cell from which the binary data is read, in accordance with a result of error correction of the binary data in the correction circuit. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0014]      FIG. 1  is a circuit diagram of a ferroelectric random access memory having an ECC circuit according to a first embodiment of the present invention;  
         [0015]      FIG. 2  is a timing chart illustrating a data read operation of the ferroelectric random access memory shown in  FIG. 1 ;  
         [0016]      FIG. 3  is a circuit diagram of a ferroelectric random access memory having an ECC circuit according to a second embodiment of the present invention;  
         [0017]      FIG. 4  is a timing chart illustrating a data read operation of the ferroelectric random access memory shown in  FIG. 3 ;  
         [0018]      FIG. 5  is a circuit diagram of a ferroelectric random access memory having an ECC circuit according to a third embodiment of the present invention; and  
         [0019]      FIG. 6  is a timing chart illustrating a data read operation of the ferroelectric random access memory shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that the drawings are schematic ones and the dimension ratios shown therein are different from the actual ones. The dimensions vary from drawing to drawing and so do the ratios of dimensions.  
       First Embodiment  
       [0021]      FIG. 1  shows a basic configuration of a semiconductor memory device according to a first embodiment of the present invention. The first embodiment is directed to a ferroelectric random access memory (FeRAM) having an ECC circuit, from which data is read by a 1T/1C system. For the sake of brevity,  FIG. 1  shows one pair of bit lines only.  
         [0022]     Referring to  FIG. 1 , a unit cell (FeRAM cell) UC serving as a memory cell is connected to one (BL 0 ) of paired bit lines (a pair of bit lines) BL. The unit cell UC nonvolatilely stores binary data such as data “0” (e.g., first potential level) and data “1” (e.g., second potential level which is higher than the first potential level).  
         [0023]     The unit cell UC includes a cell transistor CT and a ferroelectric capacitor FC. The gate electrode of the cell transistor CT is connected to a word line WL (WL 0 ) and the source terminal thereof is connected to one (BL 0 ) of the bit lines BL. One of electrodes of the ferroelectric capacitor FC is connected to the drain terminal of the cell transistor CT, and the other electrode thereof is connected to a plate line PL (PL 0 ).  
         [0024]     A reference cell RC is connected to the other bit line (/BL 0 ). The reference cell RC includes a dummy transistor dT and a dummy capacitor dC. The gate electrode of the dummy transistor dT is connected to a dummy word line DWL, and the source terminal thereof is connected to the bit line /BL 0 . One of electrodes of the dummy capacitor dC is connected to the drain terminal of the dummy transistor dT, and the other electrode thereof is connected to a dummy plate line DPL.  
         [0025]     A reset transistor RT is connected to the reference cell RC. The gate electrode of the reset transistor RT is connected to a dummy reset line DRST. The drain terminal thereof is connected to a node between the drain terminal of the dummy transistor dT and the one of the electrodes of the dummy capacitor dC. The source terminal thereof is grounded (connected to, e.g., a first potential level VSS).  
         [0026]     One unit cell UC is formed by connecting a cell transistor CT and a ferroelectric capacitor FC together in series. A plurality of unit cells UC, which are connected to one of the bit lines BL, and a plurality of reference cells RC, which are connected to the other bit line, are arranged two-dimensionally to configure a large-capacity memory cell array (not shown).  
         [0027]     A precharge circuit  11  is connected to the paired bit lines BL 0  and /BL 0 , as is a sense amplifier  12 . The precharge circuit  11  sets the potentials of the bit lines BL 0  and /BL 0  to VSS in response to the potential level of signal line BEQL. The sense amplifier  12  amplifies data supplied to the bit lines BL 0  and /BL 0  in response to the potential levels of sense amplifier driving signal lines SEN and /SEP.  
         [0028]     Local data lines LDQ 0  and /LDQ 0  are connected to the bit lines BL 0  and /BL 0  via connection selecting transistors  13   a  and  13   b , respectively. The selecting transistor  13   a  is a switch for selectively connecting the bit line BL 0  and local data line LDQ 0  to each other and disconnecting them from each other in response to the potential level of a signal line CSL. The selecting transistor  13   b  is a switch for selectively connecting the bit line /BL 0  and local data line /LDQ 0  to each other and disconnecting them from each other in response to the potential level of the signal line CSL.  
         [0029]     A “0” write circuit (“0” write buffer)  14  is connected to the local data lines LDQ 0  and /LDQ 0 . The “0” write circuit  14  transfers data to an ECC circuit (described later) and then sets to VSS the potential of the bit line BL 0  connected to the unit cell UC from which the data is read.  
         [0030]     Main data lines MDQ 0  and /MDQ 0  are connected to their respective local data lines LDQ 0  and /LDQ 0  via a pass gate circuit  15 . The pass gate circuit  15  is a switch for selectively connecting the local data line LDQ 0  and main data line MDQ 0  to each other and disconnecting them from each other in response to the potential level of DQgate and also selectively connecting the local data line /LDQ 0  and main data line /MDQ 0  to each other and disconnecting them from each other in response to the potential level of DQgate.  
         [0031]     An ECC circuit (correction circuit)  16  is connected to the main data lines MDQ 0  and /MDQ 0 . The ECC circuit  16  checks whether a plurality of data items read out of the unit cell UC include error data and correct error data if any. If error data is read out with a certain probability, the ECC circuit  16  can correct the error data, thereby improving the reliability of the memory.  
         [0032]     The ECC circuit  16  chiefly includes a parity computation circuit, a syndrome computation circuit and a data correction circuit (none of which are shown). The ECC circuit  16  compares a parity signal and data which are stored in separate memories, and detects error data in the read data. The more the amount of information (the number of data items) corresponding to a 1-bit parity signal, the lower the proportion of the parity signal to the memory capacity. More data is read out accordingly. The ECC circuit  16  corrects error data and outputs part of the error-corrected data. In burst mode, all data may be read out.  
         [0033]     In write mode, a read operation is performed before a write operation. A parity signal and read data are input to the syndrome computation circuit. The syndrome computation circuit computes a syndrome signal and then the data correction circuit corrects error data in the read data. When the amount of data corresponding to the parity signal is larger than that of write data, part of the read data is rewritten as write data and then the parity computation circuit generates a new parity signal. Part of the corrected read data, the write data, and the parity signal are written to the memory cell.  
         [0034]     In read mode, a parity signal and read data are input to the syndrome computation circuit to generate a syndrome signal. The data correction circuit checks whether the read data includes error data using the syndrome signal and corrects error data if any. The ECC circuit outputs part of the corrected error data to write back the data.  
         [0035]     An operation of the above FeRAM having an ECC circuit will be described.  FIG. 2  shows the sequence of data read operations of the FeRAM. In read mode, as shown in  FIG. 2 , the potential level of signal line BEQL of the precharge circuit  11  for precharging the potentials of bit lines BL 0  and /BL 0  with VSS is set low (L) to bring the bit lines BL 0  and /BL 0  into floating state. After that, in order to read data out of a selected unit cell UC, the potential of a corresponding word line WL 0  is set high (H) to connect the unit cell UC with the bit line BL 0 .  
         [0036]     The potential level of a reset line DRST for precharging with VSS the bit line /BL 0  side node of a reference cell RC to which a reference potential is applied is set low to bring the bit line /BL 0  into floating state. After that, the potential of the dummy word line DWL is set high to connect the reference cell RC to the bit line /BL 0 .  
         [0037]     The potential of each of the plate lines PL 0  and DPL is set at VAA (bit line amplification voltage) which is the second potential level. Thus, the voltage VAA is applied to the unit cell UC and the reference cell RC to supply data from the unit cell UC to the bit line BL 0  and supply data from the reference cell RC to the bit line /BL 0 .  
         [0038]     After that, the sense amplifier  12  compares and amplifies the potentials of the bit lines BL 0  and /BL 0  by activating the sense amplifier driving signal lines SEN and /SEP. Assuming here that the data of the unit cell UC is “1” that is the second potential level, the potential of the bit line BL 0  is VAA and that of the bit line /BL 0  is VSS.  
         [0039]     After that, the potential level of the signal line CSL becomes high, and the potential VAA of the bit line BL 0  is transferred to the local data line LDQ 0  through the selecting transistor  13   a . The potential VAA is then transferred to the ECC circuit  16  through the main data line MDQ 0 . Then, the pass gate circuit  15  that connects the local data line LDQ 0  and main data line MDQ 0  and connects the local data line /LDQ 0  and main data line /MDQ 0  is shut off (in this case the potential level of signal line DQgate is low and that of signal line /DQgate is high). Thus, the data of the unit cell UC is maintained by the ECC circuit  16 . The potential level of the signal line CSL becomes high and then the potential of each of the plate lines PL 0  and DPL becomes low.  
         [0040]     After that, the “0” write circuit  14  sets to VSS the potential of the bit line BL 0  to which the unit cell UC is connected (the potential of the bit line /BL 0  is VAA (VAA&gt;VSS)). If the data of the unit cell UC sent to the ECC circuit  16  is error data, the ECC circuit  16  outputs it as highlighted one. If not, the ECC circuit  16  writes the potential of the bit line BL 0  back to VAA and rewrites (restores) data “1” to (in) a corresponding unit cell UC.  
         [0041]     In the prior art FeRAM, the potential of the plate line PL 0  has to be maintained at a high level in data read mode. In the first embodiment, it is possible to wait for the completion of error correction in the ECC circuit  16  while the potential of the plate line PL 0  is low. Since time required for the error correction can be lengthened, data can be written back to a unit cell UC without lengthening cycle time.  
         [0042]     As described above, when the transfer of data read out of a unit cell UC to the ECC circuit  16  is completed, data “0” is temporarily written to the unit cell UC. Only in the case where data “1” needs to be written when error correction is completed in the ECC circuit  16 , data “1” is rewritten. If data “0” is written first and then data “1” is done for a data write-back operation, the results of error correction can be reflected in the data write-back operation without lengthening cycle time. Consequently, data can be written back with high efficiency, and cycle time required for the data write-back operation can be shortened.  
         [0043]     The above first embodiment is directed to a ferroelectric random access memory (FeRAM) having an ECC circuit from which data is read by a 1T/1C system. However, the present invention can be applied to a ferroelectric random access memory (FeRAM) having an ECC circuit from which data is read by a 2T/2C system.  
       Second Embodiment  
       [0044]      FIG. 3  shows a basic configuration of a semiconductor memory device according to a second embodiment of the present invention. The second embodiment is directed to a ferroelectric random access memory (FeRAM) having an ECC circuit from which data is read by a 2T/2C system. For the sake of brevity,  FIG. 3  shows one pair of bit lines only. The same components as those of  FIG. 1  are denoted by the same reference numerals and their detailed descriptions are omitted.  
         [0045]     The second embodiment differs from the first embodiment in that not a reference cell but a data cell (memory cell) is connected to one of the bit lines and no “0” write circuit is connected to a local data line. As show in  FIG. 3 , a unit cell UC serving as a memory cell is connected to one (BL 0 ) of paired bit lines BL. A data cell DC for holding complementary data is connected to the other (/BL 0 ) of the bit lines BL. The data cell DC includes a cell transistor CT and a ferroelectric capacitor FC. The gate electrode of the cell transistor CT is connected to a word line WL (/WL 0 ) and the source terminal thereof is connected to the other bit line /BL 0 . One of electrodes of the ferroelectric capacitor FC is connected to the drain terminal of the cell transistor CT, and the other electrode is connected to a plate line PL (/PL 0 ).  
         [0046]     A plurality of unit cells UC, which are connected to one of the bit lines BL and a plurality of data cells DC, which are connected to the other bit line, are arranged two-dimensionally to configure a large-capacity memory cell array (not shown).  
         [0047]     A precharge circuit  11  and a sense amplifier  12  are each connected to the bit lines BL 0  and /BL 0  as a setting circuit. Local data lines LDQ 0  and /LDQ 0  are connected to the bit lines BL 0  and /BL 0  via connection selecting transistors  13   a  and  13   b , respectively. Main data lines MDQ 0  and /MDQ 0  are connected to their respective local data lines LDQ 0  and /LDQ 0  via a pass gate circuit  15 . An ECC circuit (correction circuit)  16  is connected to the main data lines MDQ 0  and /MDQ 0 .  
         [0048]     An operation of the above FeRAM having an ECC circuit will be described.  FIG. 4  shows the sequence of data read operations of the FeRAM. In read mode, as shown in  FIG. 4 , the potential level of signal line BEQL of the precharge circuit  11  for precharging the potentials of bit lines BL 0  and /BL 0  with VSS is set low (L) to bring the bit lines BL 0  and /BL 0  into floating state. After that, in order to read data out of a selected unit cell UC, the potentials of corresponding word lines WL 0  and /WL 0  are set high (H) to connect the unit cell UC with the bit line BL 0  and the data cell DC with the bit line /BL 0 . The data cell DC connected to the bit line /BL 0  holds data complementary to the data of the unit cell UC connected to the bit line BL 0 .  
         [0049]     The potential of each of the plate lines PL 0  and /PL 0  is set at VAA (bit line amplification voltage) which is the second potential level. Thus, the voltage VAA is applied to the unit cell UC and the data cell DC to supply data from the unit cell UC to the bit line BL 0  and read data from the data cell DC to the bit line /BL 0 .  
         [0050]     After that, the sense amplifier  12  compares and amplifies the potentials of the bit lines BL 0  and /BL 0  by activating the sense amplifier driving signal lines SEN and /SEP. Assuming here that the data of the unit cell UC is “1” that is the second potential level, the potential of the bit line BL 0  is VAA and that of the bit line /BL 0  is VSS (VAA&gt;VSS).  
         [0051]     After that, the potential level of the signal line CSL becomes high, and the potential VAA of the bit line BL 0  is transferred to the local data line LDQ 0  through the selecting transistor  13   a . The potential VAA is then transferred to the ECC circuit  16  through the main data line MDQ 0 . The pass gate circuit  15  that connects the local data line LDQ 0  and main data line MDQ 0  and connects the local data line /LDQ 0  and main data line /MDQ 0  is shut off (in this case the potential level of signal line DQgate is low and that of signal line /DQgate is high). Thus, the data of the unit cell UC is maintained by the ECC circuit  16 . The potential level of the signal line CSL becomes high and then the potential of each of the plate lines PL 0  and /PL 0  becomes low.  
         [0052]     After that, unlike in the first embodiment, the sense amplifier driving signal lines SEN and /SEP are inactivated and the potential level of the signal line BEQL is set high, thereby setting the potential of each of the bit lines BL 0  and /BL 0  to VSS by the precharge circuit  11 . If the data of the unit cell UC sent to the ECC circuit  16  is error data, it is written back such that the potential of the bit line /BL 0  becomes high. If not, it is written back such that the potential of the bit line BL 0  becomes high.  
         [0053]     In the prior art FeRAM, the potentials of the plate lines PL 0  and /PL 0  have to be maintained at a high level in data read mode. In the second embodiment of the present invention, it is possible to wait for the completion of error correction in the ECC circuit  16  while the potentials of the plate lines PL 0  and /PL 0  are set low. In other words, time required for the error correction can be lengthened and thus data can be written back to a unit cell UC without lengthening cycle time.  
         [0054]     As described above, the 2T/2C data read type FeRAM having an ECC circuit according to the second embodiment allows data to be written back with high efficiency and allows cycle time for the write-back operation to be shortened. In this FeRAM, when the transfer of data read out of a unit cell UC to the ECC circuit  16  is completed, the precharge circuit  11  can temporarily write data “0” to the unit cell UC and data cell DC.  
         [0055]     In the foregoing second embodiment, the connection of the local data line LDQ 0  and main data line MDQ 0  is controlled by the pass gate circuit  15 , as is the connection of the local data line /LDQ 0  and main data line /MDQ 0 . In the 2T/2C data read type FeRAM, the pass gate circuit  15  can be omitted.  
       Third Embodiment  
       [0056]      FIG. 5  shows a basic configuration of a semiconductor memory device according to a third embodiment of the present invention. The third embodiment is directed to a ferroelectric random access memory (FeRAM) having an ECC circuit from which data is read by a 2T/2C system. For the sake of brevity,  FIG. 5  shows one pair of bit lines only. The same components as those of  FIG. 3  are denoted by the same reference numerals and their detailed descriptions are omitted.  
         [0057]     The third embodiment differs from the second embodiment in that there is no pass gate circuit for controlling a local data line and a main data line. As show in  FIG. 5 , a unit cell UC serving as a memory cell is connected to one (BL 0 ) of paired bit lines BL, and a data cell DC for holding complementary data is connected to the other (/BL 0 ) of the bit lines BL.  
         [0058]     A plurality of unit cells UC, which are connected to one of the bit lines BL, and a plurality of data cells DC, which are connected to the other bit line, are arranged two-dimensionally to form a large-capacity memory cell array (not shown).  
         [0059]     A precharge circuit  11  and a sense amplifier  12  are each connected to the bit lines BL 0  and /BL 0  as a setting circuit. Data lines DQ 0  and /DQ 0  are connected to the bit lines BL 0  and /BL 0  via connection selecting transistors  13   a  and  13   b , respectively. An ECC circuit (correction circuit)  16  is connected to the data lines DQ 0  and /DQ 0 .  
         [0060]     An operation of the above FeRAM having an ECC circuit will be described.  FIG. 6  shows the sequence of data read operations of the FeRAM. In read mode, as shown in  FIG. 6 , the potential level of signal line BEQL of the precharge circuit  11  for precharging the potentials of bit lines BL 0  and /BL 0  with VSS is set low (L) to bring the bit lines BL 0  and /BL 0  into floating state. After that, in order to read data out of a selected unit cell UC, the potentials of corresponding word lines WL 0  and /WL 0  are set high (H) to connect the unit cell UC with the bit line BL 0  and connect the data cell DC with the bit line /BL 0 . The data cell DC connected to the bit line /BL 0  holds data complementary to the data of the unit cell UC connected to the bit line BL 0 .  
         [0061]     The potential of each of the plate lines PL 0  and /PL 0  is set at VAA (bit line amplification voltage) which is the second potential level. Thus, the voltage VAA is applied to the unit cell UC and the data cell DC to read data from the unit cell UC to the bit line BL 0  and read data from the data cell DC to the bit line /BL 0 .  
         [0062]     After that, the sense amplifier  12  compares and amplifies the potentials of the bit lines BL 0  and /BL 0  by activating the sense amplifier driving signal lines SEN and /SEP. Assuming here that the data of the unit cell UC is “1” that is the second potential level, the potential of the bit line BL 0  is VAA and that of the bit line /BL 0  is VSS.  
         [0063]     After that, the potential level of the signal line CSL becomes high, and the potential VAA of the bit line BL 0  is transferred to the data line DQ 0  through the selecting transistor  13   a . The potential VAA is then transferred to the ECC circuit  16  through the data line DQ 0 . The selecting transistor  13   a  that connects the bit line BL 0  and data line DQ 0  and the selecting transistor  13   b  that connects the bit line /BL 0  and data line /DQ 0  are turned off (in this case the potential level of signal line CSL is low). Thus, the data of the unit cell UC is maintained by the ECC circuit  16 . When the potential level of the signal line CSL becomes low, the potential of each of the plate lines PL 0  and /PL 0  becomes low.  
         [0064]     Unlike in the first embodiment, the sense amplifier driving signal lines SEN and /SEP are inactivated, and the potential level of the signal line BEQL is set high, thereby setting the potential of each of the bit lines BL 0  and /BL 0  to VSS by the precharge circuit  11 . If the data of the unit cell UC sent to the ECC circuit  16  is error data, it is written back such that the potential of the bit line /BL 0  becomes high. If not, it is written back such that the potential of the bit line BL 0  becomes high.  
         [0065]     A transmission path can be disconnected from the ECC circuit  16  (the data lines DQ 0  and /DQ 0  can be disconnected from each other) even by turning off the selecting transistors  13   a  and  13   b  under the control of the signal line CSL. Even when a pass gate circuit is omitted, it is possible to wait for the completion of error correction of the ECC circuit  16  while the potentials of the bit lines BL 0  and /BL 0  are at VSS, without having an influence on the data held by the ECC circuit  16 .  
         [0066]     The above first to third embodiments are directed to one unit cell. The present invention is not limited to this, but can be applied to a so-called chain-structure FeRAM in which a plurality of unit cells are connected like a chain.  
         [0067]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.