Nonvolatile semiconductor memory device

[0002] This application is a Continuation of U.S. application Ser. No. 09/851,332 filed on May 9, 2001 now U.S. Pat. No. 6,385,087; which is a Continuation of U.S. application Ser. No. 09/505,698 filed on Feb. 17, 2000 now U.S. Pat. No. 6,252,801, which is a Continuation of U.S. application Ser. No. 09/220,328 filed on Dec. 24, 1998 now U.S. Pat. No. 6,144,582; which is a Continuation of U.S. Ser. No. 08/939,876, filed Sep. 29, 1997 now U.S. Pat. No. 5,901,083; which is a Continuation of U.S. Ser. No. 08/605,684, filed Feb. 22, 1996 now abandoned; and which is a Continuation of U.S. Ser. No. 08/179,126, filed Jan. 10, 1994 now abandoned.

[0003] 1. Field of the Invention

[0004] The present invention relates to a nonvolatile semiconductor memory device which is electrically erasable and programmable and, more particularly, to an EEPROM capable of erasing data in a small unit.

[0005] 2. Description of the Related Art

[0006] A flash EEPROM, which is one type of EEPROM and capable of electrically erasing data at once, includes a stacked memory cell transistor. The write of data to the flash EEPROM is performed by injecting channel hot electrons, and the erase of data therefrom is done by causing a Fowler-Nordheim tunnel current to flow. In this memory device, it is clearly effective to apply a negative voltage to the gate of the memory cell transistor when data is erased; therefore, a row decoder circuit for applying a negative voltage to a word line when data is erased, is required.

[0007] In the conventional EEPROM, all bits are erased at once or data is erased for each block of a large unit, but data cannot be erased for each block of a small unit.

[0008] Recently, a memory device capable of erasing data in a small unit has been developed, as is proposed in H. Kume et al., “A 3.42 μm²Flash Memory Cell Technology Conformable to a Sector Erase,” Symposium on VLSI Technology,1991, pp 77-78. In this memory device, an erase block is designated for every word line, a negative voltage is applied to only the word lines of a block to be erased, and a high voltage of, e.g., 5 V is applied to the sources of all memory cell transistors. The word lines of non-selective blocks are set in a semi-selective mode, and a positive voltage lower than the source potential is applied to the word lines in order to prevent data from being erased by mistake.

[0009] In other words, a low voltage is applied to the gate of a non-selective memory cell, and a difference in potential between the source and gate thereof is reduced, thereby preventing so-called soft erase from being caused in the non-selective memory cell.

[0010] According to the memory device described above, only the selected word line has to be set to a low level (negative voltage) and the non-selected word line has to be set to a high level (positive voltage) in the erase mode, contrary to the read and write modes, and a row decoder circuit having such a voltage setting function is therefore needed.

[0011] Conventionally, two types of row decoder circuits, that is, a read/write type positive-voltage decoder and an erase type negative-voltage decoder consisting of a P-channel MOS transistor are arranged for each word line. A negative voltage is supplied from a negative-voltage supply circuit to the respective negative-voltage decoders. The positive-voltage decoder and the negative-voltage decoder of each row are separated from each other by interposing a negative-voltage stopping P-channel MOS transistor between them.

[0012] Since the two decoders have to be arranged for each word line as described above, the number of transistors constituting the row decoder circuit is increased, with the result that the area of the row decoder circuit occupied in an integrated circuit is greatly increased and the size of a chip is also increased.

[0013] A great voltage stress is applied to the negative-voltage stopping P-channel MOS transistor. For this reason, a gate oxide film on the MOS transistor has to be made thicker than the other circuit elements, which complicates a manufacturing process.

[0014] It is an object of the present invention to provide a nonvolatile semiconductor memory device comprising a row decoder circuit which is simpler in constitution than that of a conventional device.

[0015] According to the present invention, there is provided a nonvolatile semiconductor memory device comprising:

[0016] a memory cell array including a plurality of memory cells arranged in rows and columns and each having a transistor having a source, a drain and a gate and capable of electrically erasing and rewriting data;

[0017] a plurality of word lines to which gates of the plurality of memory cells arranged in the same row of the memory cell array are connected in common;

[0018] a plurality of bit lines to which drains of the plurality of memory cells arranged in the same column of the memory cell array are connected in common; and

[0019] word line selection means for selecting one of the plurality of word lines in accordance with address information, applying a first voltage having a negative value to the selected one of the plurality of word lines in a data erase mode, and applying a second voltage of a positive value to each of non-selected word lines.

[0020] Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

[0021] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0022] FIG. 1 is a block diagram showing the entire constitution of an EEPROM according to a first embodiment of the present invention;

[0023] FIG. 2 is a circuit diagram showing a constitution of part of a memory cell array and that of part of a row decoder circuit both included in the EEPROM of FIG. 1;

[0024] FIG. 3 is a circuit diagram showing in detail a main decoder circuit of the row decoder circuit of FIG. 2;

[0025] FIG. 4 is a cross-sectional view showing a device structure of part of the main decoder circuit of FIG. 3;

[0026] FIG. 5 is a circuit diagram of the EEPROM shown in FIG. 3 which data is erased;

[0027] FIG. 6 is a circuit diagram showing in detail part of a predecoder circuit of the row decoder circuit of FIG. 2;

[0028] FIG. 7 is a circuit diagram showing a constitution of part of the predecoder circuit of FIG. 6;

[0029] FIG. 8 is a circuit diagram showing a constitution of part of a modification to the EEPROM according to the first embodiment;

[0030] FIG. 9 is a circuit diagram showing a constitution of part of an EEPROM according to a second embodiment of the present invention; and

[0031] FIG. 10 is a circuit diagram showing a constitution of an EEPROM according to a third embodiment of the present invention.

[0032] Embodiments of the present invention will now be described, with reference to the accompanying drawings.

[0033] FIG. 1 shows an EEPROM according to a first embodiment of the present invention. The EEPROM comprises a memory cell array 1, row decoder circuit 2, a column decoder circuit 3, a mode setting signal generation circuit 4, an erase negative-voltage generation circuit 5, a write high-voltage generation circuit 6, a read intermediate-voltage generation circuit 7, mode selection circuits 8A and 8B, and a source decoder circuit 9.

[0034] The memory cell array 1 includes a number of stacked memory cell transistors each of which has a floating gate and a control gate and which are arranged in rows and columns, though they are not shown. The memory cell array 1 is divided into plural cell blocks (four cell blocks in the first embodiment), and the memory cell transistors of each cell block have a source in common.

[0035] The row decoder circuit 2 selects memory cell transistors of one row from all the memory cell transistors arranged in the memory cell array 1 in accordance with a row address. Similarly, the column decoder circuit 3 selects memory cell transistors of one column therefrom in accordance with a column address.

[0036] The mode setting signal generation circuit 4 generates a mode setting signal according to an operation mode of the EEPROM. The operation mode includes a data erase mode, a data write mode, and a data read mode.

[0037] The erase negative-voltage generation circuit 5, the write high-voltage generation circuit 6, and the read intermediate-voltage generation circuit 7 generate an erase negative voltage VEE, a write high voltage VPP, and a read intermediate voltage VRD, respectively, and these voltage are used in the memory cell array in the data erase, data write, and data read modes, respectively.

[0038] The mode selection circuits 8A and 8B receive the mode setting signal from the mode setting signal generation circuit 4, and selects their respective voltages to be used in the row decoder circuit 2 and column decoder circuit 3 in the data erase, data write, and data read modes.

[0039] The source decoder circuit 9 applies a predetermined voltage to the source common to the memory cell transistors of each cell block in the memory cell array 1.

[0040] In addition, the EEPROM includes a voltage generation circuit for generating a voltage which is lower than a power supply voltage to be applied to the gate of a non-selected memory cell transistor in the data erase mode.

[0041] FIG. 2 shows part of one cell block of the memory cell array 1 as well as the row decoder circuit 2 and source decoder circuit 9. The cell block includes a number of stacked memory cell transistors 11 arranged in rows and columns. The control gates of the memory cells 11 arranged in the same row are connected in common to each of word lines 12. The drains of the memory cells 11 arranged in the same column are connected in common to each of bit lines 13. The sources of the memory cell transistors 11 are connected in common to a source line 14, and a voltage is applied from the source decoder circuit 9 to the source line 14.

[0042] The row decoder circuit 2 comprises a predecoder circuit 21 to which some of row addresses are input, a predecoder circuit 22 to which some of the row addresses other than those input to the circuit 21 are input, and a main decoder circuit 23 to which output signals of the predecoder circuits 21 and 22 are supplied. In addition to a normal power supply voltage VCC of 5 V, a negative voltage EE of, e.g., about −10 V generated by the erase negative-voltage generation circuit 5, a high voltage VPP of e.g., about 12 V generated by the write high-voltage generation circuit 6, a voltage VWL of, e.g., 3 V, which is lower than the power supply voltage VCC and is applied to the gate of a non-selected memory cell transistor in the erase mode, and a ground voltage VSS of 0 V, are applied to the row decoder circuit 2. The above voltages VCC, VWL and VSS are also applied to the source decoder circuit 9, and one of these voltages is output to the source line 14 in the corresponding cell block.

[0043] In the memory cell array 1, in the read mode, the power supply voltage VCC (5 V) is applied to a selected one of the word lines (selected word line) 12, and a read intermediate voltage VRD of, e.g., about 1 V, which is generated by the read intermediate-voltage generation circuit 7, is applied to a selected one of the bit lines (selected bit line) 13. In the write mode, the write high voltage VPP (12 V) is applied to the selected word line 12, and a high voltage is applied to the selected bit line 13, too. In the erase mode, the power supply voltage VCC is applied to the source line 14, the negative voltage VEE is applied only to the selected word line 12, and the voltage VWL (3 V) is applied to a non-selected word line. The bit lines 13 are rendered in, for example, a floating state.

[0044] The main decoder circuit 23 of the row decoder circuit 2 decodes outputs of the predecoder circuits 21 and 22, and applies predetermined voltages to the selected word line and non-selected word line, respectively. FIG. 3 shows a circuit arrangement of part of the main decoder circuit 23.

[0045] The main decoder circuit 23 includes decoder circuits 24 for every word line group having the same number of word lines. Each of the decoder circuits 24 includes a CMOS type NAND gate 31 supplied with decode signals which are different combinations of signals output from the predecoder circuit 22, a CMOS type inverter 32 for inverting an output signal of the NAND gate 31, N-channel MOS transistors 33 and 34 and P-channel MOS transistors 35 and 36 which correspond to each of the word lines 12 within the word line group.

[0046] The source-to-drain current paths of the N- and P-channel MOS transistors 33 and 35 corresponding to the same word line 12 are connected in parallel to each other. One end of each of the source-to-drain current paths is supplied with a signal output from its corresponding one of plural decode output terminals of the predecoder circuit 21, and the other ends thereof are connected to their corresponding word line 12. The decode output terminals of the predecoder circuit 21 output the voltage VCC or VPP when a word line corresponding to the read or write mode is selected, and output the ground voltage of 0 V when it is not selected. Further, the decode output terminals output the voltage VEE when a word line corresponding to the erase mode is selected, and output the voltage VWL when it is not selected.

[0047] The source-to-drain current paths of the other N- and P-channel MOS transistors 34 and 36 are also connected in parallel to each other. One end of each of the source-to-drain current paths is connected to a node to which a predetermined voltage SWL is applied, and the other ends thereof are connected to their corresponding word line 12.

[0048] The voltage SWL is changed to the voltage VWL in the erase mode, and to the ground voltage VSS in the other modes.

[0049] The gates of the P-channel MOS transistors 35 connected to the respective word lines 12 are supplied with a common output signal of the NAND gate 31, and the gates of the N-channel MOS transistors 33 are supplied with a common output signal of the inverter 32. Similarly, the gates of the N-channel MOS transistors 34 connected to the respective word lines 12 are supplied with a common output signal of the NAND gate 31, and the gates of the P-channel MOS transistors 36 are supplied with a common output signal of the inverter 32. In other words, each N-channel MOS transistor 33 and each P-channel MOS transistor 35 constitute a CMOS transfer gate (CMOS switch) T1, and each N-channel MOS transistor 34 and each P-channel MOS transistor 36 constitute a CMOS transfer gate (CMOS switch) T2.

[0050] The two transfer gates T1 and T2 are so controlled as to supply the decode signal or voltage SWL from the predecoder circuit 21 to their corresponding word line 12 in response to the output signal of the NAND gate 31.

[0051] FIG. 4 is a cross-sectional view showing a device structure of part of the memory cell array 1 shown in FIG. 2 which includes the N-channel MOS transistor 33 and P-channel MOS transistor 36 shown in FIG. 3. The ground voltage VSS of 0 V is applied to a P-type semiconductor substrate 40 of the device structure, and N wells 41 and 42 are formed in the substrate 40. These N wells can be formed simultaneously by the same process or separately by different processes. For example, the N well 41 is formed first to have a great junction depth xj, and then a P well 43 is formed therein. Such a double well structure of the N and p wells 41 and 43 allows the P well 43 to be electrically separated from the P-type semiconductor substrate 40.

[0052] The N-channel MOS transistor 33 is formed in the P well 43. More specifically, the source 44 and drain 45 of the N-channel MOS transistor 33 are formed in the P well 43, and the gate 46 thereof is formed above the surface of the P well 43. Various voltages which includes a negative voltage and are output from the output terminals of the predecoder circuit 21 shown in FIG. 2, are applied to the source 44 of the N-channel MOS transistor 33. The drain 45 is connected to its corresponding one of the word lines 12. The gate 46 is supplied with an output of the inverter 32 shown in FIG. 3. The negative voltage VEE is applied to the P well 43 in the erase mode, and the ground voltage of 0 V is applied thereto in the other modes.

[0053] The P-channel MOS transistor 36 is formed in the N well 42. More specifically, the source 46 and drain 47 of the P-channel MOS transistor 36 are formed in the N well 42, and the gate 48 thereof is formed above the surface of the N well 42. The power supply voltage SWL is applied to the source 46 and N well 42. The drain 47 is connected to the word line 12 to which the drain 45 of the N-channel MOS transistor 33 is connected. The gate 48 is supplied with an output of the inverter 32 shown in FIG. 3.

[0054] The N- and P-channel MOS transistors 34 and 35 shown in FIG. 3 have the same structures as the above-described N- and P-channel MOS transistor 33 and 36 do, respectively.

[0055] The memory cell transistor 11, NAND gate 31, and N-channel MOS transistor are formed in the P-type semiconductor substrate 40. The memory cell transistor 11 is formed in the substrate 40 in order to prevent the substrate potential of the memory cell transistor 11 from being floated when a substrate current flows in the write mode.

[0056] An operation of the main decoder circuit 23 including the decoder circuits 24, shown in FIG. 3, will now be described.

[0057] In the read, write, and erase modes, the NAND gate 31 in the decoder circuit 24 to which a word line group including a selective word line 12 is connected, is supplied with high-level decode signals from the corresponding predecoder circuit 22, and the output signal of the NAND gate 31 becomes low in level. Moreover, the output signal of the inverter 32 to which the output signal of the NAND gate 31 is input, becomes high in level. Thus, each transfer gate T1 connected to its corresponding word line 12, is turned on.

[0058] At least one of the decode signals supplied from the predecoder circuit 22 to the NAND gates 31 in the other decoder circuits 24, is at a low level. Therefore, in the decoder circuits 24 to which the word line groups including no selective word line 12 are connected, the output signal of the NAND gate 31 becomes high, and that of the inverter 32 becomes low, with the result that the transfer gates T2 are turned on in these decoder circuits 24.

[0059] In the read mode, the predecoder circuit 21 for applying a voltage to one end of each transfer gate T1 outputs the voltage VCC of 5 V only from its output terminal corresponding to the selective word line 12 based on an address, and outputs the ground voltage VSS of 0 V from its output terminal corresponding to the non-selective word lines 12. In other words, the voltage VCC of 5 V is applied only to the selective word line 12, and the voltages VSS of 0 V is applied to the other non-selective word lines 12. In this time, the voltage SWL applied to each transfer gate T2 is changed to the ground voltage VSS of 0 V, and the decoder circuits 24 connected to the word line groups not including the selective word line, apply the ground voltage VSS of 0 V to the respective word lines through the transfer gates T2.

[0060] In the write mode, the predecoder circuit 21 outputs the high voltage VSS of 12 V from its output terminal corresponding to the selective word line 12, and outputs the ground voltage VSS of 0 V from its output terminal corresponding to the non-selective word lines 12. In other words, the voltage of 12 V is applied only to the selective word line 12, and the ground voltage VSS of 0 V is applied to the other non-selective word lines 12. In this time, the voltage SWL applied to each transfer gate T2 is changed to the ground voltage VSS of 0 V, and the decoder circuits 24 connected to the word line groups not including the selective word line, apply the ground voltage VSS of 0 V to the respective word lines through the transfer gates T2.

[0061] In the erase mode, the predecoder circuit 21 outputs the negative voltage VEE of −10 V from its output terminal corresponding to the selective word line 12, and outputs the voltage VWL of 3 V from its output terminal corresponding to the non-selective word lines 12. Furthermore, the transfer gates T2 of the decoder circuits 24 to which the word line groups not including the selective word line, are turned on and, in this time, the power supply SWL connected to the transfer gates T2 is set to the voltage VWL of 3 V. In other words, the decoder circuits 24 to which the word line groups not including the selective word line 12, apply the voltage VWL of 3 V to the word lines 12. Further, in the erase mode, the voltage VCC of 5 V is output from the source decoder circuit 9.

[0062] An operation of the EEPROM in the erase mode will be described, with reference to FIG. 5. As shown in FIG. 5, the voltage VEE of −10 V is applied to the control gate of one of the plural memory cell transistors 11 which is connected to the selected word line 12, while the voltage VWL of 3 V is applied to the control gates of the other memory cell transistors connected to the non-selected word lines 12. The voltage VCC of 5 V is applied to the source line 14.

[0063] In a selected memory cell transistor (selected cell) whose control gate is supplied with the voltage VEE of −10 V, the potential of the floating gate is set to a negative value corresponding to the voltage VEE, by the voltage applied to the control gate, the Fowler-Nordheim tunnel channel is caused to flow between the floating gate and source by applying a great electric field between them, and the electrons stored in advance in the floating gate are emitted to the source, thereby erasing data.

[0064] In the non-selected memory cell transistors (non-selected cells), the voltage of 3 V and the voltage of 5 V are applied to the control gate and the source, respectively, and the electric field generated between the floating gate and control gate is weaker than in the selected memory cell. For this reason, no soft erase occurs in the non-selected cells.

[0065] The circuit arrangement of the predecoder circuit 21, which applies various types of voltages to the transfer gates T1 shown in FIG. 3, will now be described in detail.

[0066] The predecoder circuit 21 has output terminals which are equal in number to the word lines of each word line group, and also has decode/level conversion circuits which are equal in number to the output terminals. FIG. 6 shows a decode/level conversion circuit corresponding to one of the output terminals of the predecoder circuit 21. In FIG. 6, reference numeral 51 denotes a decoder for decoding an address, and reference numeral 52 indicates a NAND gate to which a plurality of address signals is input. By using an inverter 53 and two transfer gates 54 and 55 whose conduction is controlled in response to erase mode signals “erase” and “{overscore (erase)}”, the outputs of the NAND gate 52 are supplied from the decoder 51 as complementary signals whose logical levels are inverted in the erase mode and the other modes. The levels of VOC (5 V) signals output from the decoder 51 are changed in sequence to different ones by two level shifters 56 and 57 and two CMOS inverters 58 and 59, and the signals whose levels have been changed, are supplied to the main decoder circuit 23 shown in FIG. 3.

[0067] The level shifters 56 and 57 are each constituted by P-channel MOS transistors 61 and 62 and N-channel MOS transistors 63 and 64. A voltage SWWL is applied to the sources of the P-channel MOS transistors 61 and 62 of the level shifter 56, and a ground voltage VSS is applied to the sources of the N-channel MOS transistors 63 and 64 thereof. On the other hand, a voltage SWWL is applied to the sources of the P-channel MOS transistors 61 and 62 of the level shifter 57, and a voltage SBB is applied to the sources of the N-channel MOS transistors 63 and 64 thereof. Furthermore, a voltage SWWL1 and voltage SBB are applied to the CMOS transistors 58 and 59. The voltage SWWL is changed to the voltage VCC (5 V) in the read mode and to the voltage VPP (12 V) in the write mode. The voltage SBB is usually the voltages VSS (0 V) and changed to the voltage VEE (−10 V) in the erase mode. The voltage SSWW1 is basically equal to the voltage SWWL and changed to the voltage of 3 V in the erase mode.

[0068] Consequently, the levels of the VCC signals decoded by the decoder 51, are changed in sequence to SWWL to VSS voltages by the level shifter 56, to SWWL to SBB voltages by the level shifter 57, and to SWWL to SBB voltages by the inverters 58 and 59.

[0069] The predecoder circuit 22 shown in FIG. 2, which supplies a signal to the NAND gate 31 in the decoder circuit 24 shown in FIG. 3, has a plurality of output terminals, and also has decode/level conversion circuits which are equal in number to the output terminals. FIG. 7 shows a circuit arrangement of one of the decode/level conversion circuits, which is the same as shown in FIG. 6, except that a NAND gate 71 is formed in place of the decoder 51 shown in FIG. 6. The predecoder circuit 22 always outputs a high-level voltage from a selected one of the output terminals and a low-level voltage from the non-selected output terminals. These high- and low-level voltages are, as in the case with FIG. 6, output as SWWL1 to SBB voltages, by using two level shifters 56 and 57 and two CMOS inverters 58 and 59. It is thus necessary to operate the NAND gate 31 and inverter 32 shown in FIG. 3 with the SWWL1 and SBB voltages.

[0070] In the EEPROM having the above arrangement, data can be erased for each word line by applying a negative voltage to a single word line in the erase mode and, unlike the conventional EEPROM, two different row decoders of the positive and negative voltage decoders need not be arranged. Furthermore, unlike the conventional EEPROM, the positive and negative voltage decoders need not be separate for each other in each row by interposing the negative voltage stopping P-channel MOS transistor between them. As a result, the number of the transistors constituting the row decoder circuit can be decreased, the area of the row decoder circuit occupied in an integrated circuit can be reduced, and the size of a chip can also be reduced, as compared with the conventional EEPROM.

[0071] In the above embodiment, the single word line 12 is selected, and data is erased for each row. If the row decoder circuit is modified so that all the word lines in the call block are selected at the same time, data can be erased in a larger unit.

[0072] FIG. 8 is a circuit diagram showing a modification to the EEPROM according to the first embodiment described above. In this EEPROM, a NAND gate 72 is used in place of the decoder 51 shown in FIG. 6. The NAND gate 72 is supplied with an erase mode signal {overscore (erase)} as well as an address signal. In the EEPROM shown in FIG. 8, the output signals of the predecoder circuit 21 are all changed to negative voltages, the negative voltage are applied to all the word lines in the call block, thereby erasing data in the cell block at once.

[0073] FIG. 9 is a circuit diagram showing a constitution of part of a main decoder circuit of an EEPROM according to a second embodiment of the present invention. In the above first embodiment, the source-to-drain current paths of the N- and P-channel MOS transistors 34 and 36 are connected in parallel to each other, one end of each of the source-to-drain current paths is connected to its corresponding word line, and the other ends of these current paths are connected in common to the voltage SWL. According to the second embodiment, one end of each source-to-drain current path of the N- and P-channel MOS transistors 34 and 36 is connected to its corresponding word line 12, the other end of the source-to-drain current path of the N-channel MOS transistor 34 is connected to the voltage SBB, and the other end of the source-to-drain current path of the P-channel MOS transistor 36 is connected to the voltage SWWL1.

[0074] FIG. 10 is a circuit diagram showing a constitution of part of the main decoder circuit of an EEPROM according to a third embodiment of the present invention. In the third embodiment, if, as shown in FIG. 10, a latch circuit 73 controlled in response to a set signal SET and a reset signal RESET is arranged on the output side of the NAND gate 31 in the main decoder circuit shown in FIG. 3, and the latch circuit 73 is set to erase data before the data is actually erased, the data can be erased by applying negative voltages to an arbitrary number of word lines 12 at the same time.

[0075] As described above, in the nonvolatile semiconductor memory device according to the present invention, the constitution of the row decoder circuit can be made simpler than that of the row decoder circuit of the conventional device.

[0076] 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 devices 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.