Abstract:
A circuit configured for providing hot-carrier effect protection, the circuit comprising a first transistor including a first terminal and a second terminal, the first terminal being coupled to a conductive pad, a switch device including a terminal coupled to the conductive pad, and a control circuit configured for keeping the switch at an off state during a receiving mode at which a signal of a first voltage level or a reference level is received at the conductive pad, keeping the switch at the off state during a transmitting mode from which a signal of a second voltage level or the reference level is transmitted at the conductive pad, and keeping the switch at an on state during a transition from the receiving mode when receiving a signal of the first voltage level to the transmitting mode when transmitting a signal having the reference voltage level, wherein during the transition a voltage across the first terminal and the second terminal of the first transistor is maintained at a level below approximately the first voltage level minus the second voltage level.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/823,453, filed Oct. 6, 2006. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to circuits and methods for protecting a circuit or buffer, and more particularly, to a circuit configured for preventing the hot-carrier effect in a mixed-voltage input/output (I/O) buffers. 
         [0003]    With the progress in complementary metal-oxide-semiconductor (CMOS) manufacturing technologies, the dimensions of transistors have been scaled down to reduce the silicon cost as well as to meet the increasing demands for more reliable circuit performance and faster operating speed. The thinner gate oxide of a CMOS transistor helps reduce the core power supply voltage (VDD) and therefore achieves lower power consumption. However, the maximum tolerable voltage across the transistor terminals (drain, source, gate and bulk) should be decreased accordingly to ensure the lifetime of the CMOS transistor. 
         [0004]    As an illustrative example, with back compatibility to earlier defined standards or interface protocols of CMOS integrated circuits (ICs) in a microelectronics system, the chips fabricated in advanced CMOS processes operating in the VDD domain may receive input signals with voltage levels (VDDH) higher than VDD. For example, the VDDH and VDD voltage levels are approximately 3.3 volts (V) and 1.5V, respectively, in the Peripheral Component Internet Extended (PCI-X) 2.0 applications. Such mixed-voltage input/output (I/O) interfaces must be designed to overcome several problems, such as gate-oxide reliability, hot-carrier degradation and undesired circuit leakage paths between chips. The hot-carrier induced degradation, among others, has become one of the reliability concerns in metal-oxide-semiconductor field effect transistor (MOSFET) devices fabricated in deep sub-micron technologies, which feature short channel length and high electric field. The hot-carrier effect refers to a phenomenon that carriers are accelerated by channel electric fields and become trapped in a gate oxide. The hot-carrier effect may incur deviation of threshold voltage (V th ), undesirable transconductance (g m ), and linear (I DLIN ) and saturation (I DSAT ) drain currents, resulting in degradation or even failure of a transistor. 
         [0005]      FIG. 1  is an exemplary simplified circuit diagram of a buffer  10  in a mixed-voltage interface. Referring to  FIG. 1  as an example, the buffer  10  includes a pre-driver  11 , a post-driver or output circuit  12 , an input circuit  13  and an I/O pad  14 . The pre-driver  11  and the input circuit  13  are simplified into function blocks for convenience. The pre-driver  11  generates control signals PU and PD in response to an output enable (OE) signal and a data output (Dout) signal from an internal circuit (not shown), respectively. The post-driver  12  further includes a pull-up network  121  and a pair of stacked transistors MN 0  and MN 1 , which are thin-oxide devices tolerant of the VDDH level. The pull-up network  121 , which is simplified into a function block, includes a terminal for receiving the control signal PU. The gates of the transistors MN 0  and MN 1  are respectively connected to the VDD line and the pre-driver  11  to receive the control signal PD. The buffer  10  receives input signals of the VDDH level through the I/O pad  14  to the input circuit  13 , and transmits output signals of the VDD level from an input terminal D out  to the I/O pad  14 . During the transition from the receiving operation to the transmitting operation, the transistor MN 0  may be susceptible to the hot-carrier effect, which will be discussed in detail. 
         [0006]      FIG. 2  is a schematic cross-sectional view of an n-type metal-oxide-semiconductor (NMOS) transistor  20 . Referring to  FIG. 2 , the NMOS transistor  20  includes a heavily doped n-type source  20 -S and a heavily doped n-type drain  20 -D formed in a p-type substrate, a thin layer of silicon dioxide  20 -O grown over the substrate, and a conductive gate material  20 -G formed over the dioxide  20 -O between the source  20 -S and the drain  20 -D. The source  20 -S is connected to ground. In operation, the gate-to-source voltage may modify the conductance of a region under the gate  20 -G, allowing a gate voltage to control a current following between the source  20 -S and the drain  20 -D. When positive voltages, V G  and V D , are applied to the gate  20 -G and the drain  20 -D, respectively, an inversion layer is produced as the V G  is equal to or larger than the threshold voltage (V th ) of the NMOS transistor  20 . When the value of V D  is increased, the induced conducting channel narrows at the drain end. The induced electron charge at the drain end approaches zero as V D  approaches (V G −V th ). That is, the channel is no longer connected to the drain  20 -D when V D  is greater than (V G −V th ), which is known as pinch-off. At this time, the electric field may start to rise dramatically at the pinch-off point of the NMOS transistor  20 . In the high electric field, carriers are accelerated to high velocities, reaching a maximum kinetic energy (hot) near the drain  20 -D. If the carrier energy is high enough, impact ionization can occur, creating electron-hole pair  21 . The generated electrons in the electron-hole pair  21  called secondary electrons tend to be swept to the drain  20 -D. Furthermore, the generated holes in the electron-hole pair  21  called secondary holes may be swept into the substrate in the NMOS transistor  20 . 
         [0007]    Some of the electrons generated in the space charge region are attracted to the oxide  20 -O due to the electric field induced by the positive gate voltage, V G . These generated electrons have energies far greater than the thermal-equilibrium value and are called hot electrons (or hot carriers)  22 . If the hot electrons  22  have energies on the order of 1.5 electron volt (eV), they may be able to tunnel into the oxide  20 -O. In some cases the generated holes and electrons can attain enough energy to surmount the Si—SiO 2  barrier and become trapped in the gate oxide  20 -O. The charge trapping in interface states may disadvantageously cause a shift in the threshold voltage, additional surface scattering, and reduced mobility. The hot electron charging effects are continuous processes, so the NMOS transistor  20  degrades over a period of time. 
         [0008]    Referring again to  FIG. 1 , the hot-carrier induced degradation or gate-oxide reliability in the VDDH-tolerant I/O buffer  10  may exist in the following two states: (1) the state of receiving VDDH input signals, and (2) the state of a transition from receiving VDDH input signals to transmitting 0-V output signals. When the VDDH-tolerant I/O buffer  10  receives VDDH input signals, the PU and PD signals are kept at VDD and 0 V, respectively, to disable the output circuit  12 . Since the transistor MN 1  is turned off, the transistor MN 0  is weakly turned “on”, which results in a voltage of about VDD at the source of the transistor MN 0 . In each of the stacked transistors MN 0  and MN 1 , the voltages drop across the gate-oxide and the drain-source are both lower than or equal to the supply voltage (VDD). Therefore, there is neither hot-carrier degradation nor gate-oxide overstress issue in the mixed-voltage I/O buffer  10  when receiving VDDH input signals. 
         [0009]    During a transition from receiving VDDH input signals to transmitting 0-V output signals, the I/O pad  14  originally has a voltage of VDDH before being pulled down. At this transition moment, the transistor MN 1  is turned on by the PD signal from the pre-driver  11 , and the transistor MN 0  is subsequently switched on when its source is pulled down by the transistor MN 1 . The voltage at the drain of the transistor MN 1  may be approximated as the saturation drain voltage (V DSAT ). For example, the voltage at the source of the transistor MN 0  is approximately 0.5V in a 0.18-μm CMOS process. Since the original VDDH voltage at the I/O pad  14  is not pulled down immediately, the drain-to-source voltage of the transistor MN 0  is greater than the normal supply voltage (VDD) during this transition, which results in the significant hot-carrier degradation in the transistor MN 0 . 
         [0010]    It may therefore be desirable to have a circuit that may protect a circuit, such as an I/O buffer, from the hot-carrier effect. It may also be desirable to have a buffer circuit that may be immune from the hot-carrier effect. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    Examples of the present invention may provide a circuit configured for providing hot-carrier effect protection, the circuit comprising a first transistor including a first terminal and a second terminal, the first terminal being coupled to a conductive pad, a switch device including a terminal coupled to the conductive pad, and a control circuit configured for keeping the switch at an off state during a receiving mode at which a signal of a first voltage level or a reference level is received at the conductive pad, keeping the switch at the off state during a transmitting mode from which a signal of a second voltage level or the reference level is transmitted at the conductive pad, and keeping the switch at an on state during a transition from the receiving mode when receiving a signal of the first voltage level to the transmitting mode when transmitting a signal having the reference voltage level, wherein during the transition a voltage across the first terminal and the second terminal of the first transistor is maintained at a level below approximately the first voltage level minus the second voltage level. 
         [0012]    Some examples of the present invention may also provide a circuit configured for providing hot-carrier effect protection, the circuit comprising a conductive pad at which a signal of a first voltage level or a reference level is received during a receiving mode and from which a signal of a second voltage level or the reference voltage level is transmitted, a first transistor including a first terminal and a second terminal, the first terminal being coupled to the conductive pad, and a control circuit configured for maintaining a voltage across the first terminal and the second terminal of the first transistor at a level below approximately the first voltage level minus the second voltage level during a transition from the receiving mode when receiving a signal of the first voltage level to the transmitting mode when transmitting a signal of the reference level. 
         [0013]    Examples of the present invention may further provide a circuit configured for providing hot-carrier effect protection, the circuit comprising a conductive pad at which a signal of a first voltage level or a reference level is received during a receiving mode and from which a signal of a second voltage level or the reference voltage level is transmitted, a first transistor including a first terminal and a second terminal, a second transistor including a third terminal coupled to the first terminal and a fourth terminal coupled to the conductive pad, and a first control circuit configured for maintaining a voltage across the first terminal and the second terminal of the first transistor at a level below approximately the first voltage level minus the second voltage level during a first transition from the receiving mode when receiving a signal of the first voltage level to the transmitting mode when transmitting a signal of the second voltage level. 
         [0014]    Examples of the present invention may also provide a method of protecting a circuit from hot-carrier effect protection, the method comprising providing a first transistor including a first terminal and a second terminal, coupling the first terminal of the first transistor to a conductive pad, providing a switch device including a terminal, coupling the terminal of the switch device to the conductive pad, keeping the switch at an off state during a receiving mode at which a signal of a first voltage level or a reference level is received at the conductive pad, keeping the switch at the off state during a transmitting mode from which a signal of a second voltage level or the reference level is transmitted at the conductive pad, keeping the switch at an on state during a transition from the receiving mode when receiving a signal of the first voltage level to the transmitting mode when transmitting a signal having the reference voltage level, and maintaining a voltage across the first terminal and the second terminal of the first transistor at a level below approximately the first voltage level minus the second voltage level during the transition. 
         [0015]    Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
         [0016]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings examples which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0018]    In the drawings: 
           [0019]      FIG. 1  is a simplified circuit diagram of a buffer in a mixed-voltage interface; 
           [0020]      FIG. 2  is a schematic cross-sectional view of an n-type metal-oxide-semiconductor (NMOS) transistor; 
           [0021]      FIG. 3A  is a schematic block diagram of a buffer circuit in a mixed-voltage interface consistent with an example of the present invention; 
           [0022]      FIG. 3B  is an exemplary circuit diagram of the buffer circuit illustrated in  FIG. 3A ; 
           [0023]      FIG. 4  is a schematic circuit diagram of a buffer circuit consistent with an example of the present invention; and 
           [0024]      FIGS. 5A to 5C  are plots illustrating simulation results of the buffer circuit illustrated in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Reference will now be made in detail to the present examples of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0026]      FIG. 3A  is a schematic block diagram of a buffer circuit  30  in a mixed-voltage interface consistent with an example of the present invention. Referring to  FIG. 3A , the buffer circuit  30  may include a pre-driver  31 , a post-driver or output circuit  32 , an input circuit  33 , an input/output (I/O) pad  34 , a tracking circuit  35  and a switch device  36 . The post-driver  32  may further include a pull-up network  321  and stacked NMOS transistors MN 0  and MN 1 . The pre-driver  31 , the pull-up network  321  and the input circuit  33  are simplified into function blocks for convenience. The pre-driver  31  generates control signals PU and PD in response to an output enable (OE) signal and a data output (Dout) signal from an internal circuit (not shown), respectively. The stacked transistors MN 0  and MN 1  are, for example, thin-oxide devices tolerant of a VDDH level. The pull-up network  321  includes a terminal (not numbered) for receiving the control signal PU. The transistor MN 0  includes a gate (not numbered) connected to a VDD line, and the transistor MN 1  includes a gate (not numbered) connected to the pre-driver  31  to receive the control signal PD through a delay cell  37 . The buffer circuit  30  may operate in a receiving mode to receive input signals of the VDDH level to the input circuit  33  through the I/O pad  34 , and may operate in a transmitting mode to transmit output signals of the VDD level through the post-driver  32  to the I/O pad  34 . The VDDH voltage level is greater than the VDD level. In one example, the VDDH and VDD voltage levels are approximately 3.3V and 1.5V, respectively. 
         [0027]    The tracking circuit  35  includes a first terminal (not numbered) coupled to the OE signal, a second terminal (not numbered) connected to the I/O pad  34 , and a third terminal (not numbered) connected to the switch device  36 . The tracking circuit  35  is configured for generating a control signal V CTRL  in response to the OE signal to control the state of the switch device  36 . During a transition from receiving VDDH input signals to transmitting 0-V output signals, the switch device  36  is turned on by the control signal V CTRL  to pull down the voltage level at the I/O pad  34  to VDD. The delay cell  37  provides a delay long enough to have the I/O pad  34  pulled down to VDD before the transistor MN 1  is turned on by the control signal PD. Thus, the drain-to-source voltage of the transistor MN 0  during the transition may not exceed its maximum normal operation voltage range (VDD), which prevents the transistor MN 0  from the hot-carrier degradation. The switch device  36  is kept off in the receiving and transmitting modes and thus does not interfere with the normal operation of the buffer circuit  30  in both the receiving and transmitting modes. The switch device  36  is not switched on until there is a transition from receiving an input VDDH signal to transmitting an output 0-V signal. 
         [0028]      FIG. 3B  is a circuit diagram of the buffer circuit  30  illustrated in  FIG. 3A . For the purpose of convenience, the input circuit  33  illustrated in  FIG. 3A  is omitted. Referring to  FIG. 3B , the tracking circuit  35  may include a level shifter  351 , an NMOS transistor MN 2  and PMOS transistors MP 1  and MP 2 . The level shifter  351  is configured for shifting a ground voltage level to the VDD level in response to the OE signal in the receiving mode, and shifting the VDD level to the VDDH level in response to the OE signal in the transmitting mode. The transistor MN 2  includes a gate connected to the level shifter  351 , a drain connected to VDD, and a source connected to a gate of the transistor MP 0 . Skilled persons in the art will understand that the source and drain of a MOS transistor are exchangeable, depending on the voltage levels applied thereto. The transistor MP 2  includes a gate connected to the I/O pad  34 , a source connected to VDD, and a drain connected to the gate of the transistor MP 0 . The transistor MP 1  includes a gate connected to the VDD, a source connected to the I/O pad  34 , and a drain connected to the gate of the transistor MP 0 . 
         [0029]    The switch device  36  may include a PMOS transistor MP 0  further including a gate connected to the source of the transistor MN 2 , a source connected to the VDD, and a drain connected to the I/O pad  34 . 
         [0030]    The delay cell  37  may include an inverter chain  371 . In one example according to the present invention, the delay cell  37  further includes a capacitor  372  between the output of the inverter chain  371  and the gate of the transistor MN 1  to provide a desirable delay time. The desirable delay time Δt may be estimated in an equation below. 
         [0000]      ΔQ=C L ΔV=I 36 Δt 
         [0031]    wherein C L  is an output loading, ΔV is the difference between VDDH and VDD, i.e., VDDH-VDD, and  136  is a driving current of the switch device  36 . 
         [0032]    When the buffer circuit  30  operates in the receiving mode, the output enable signal OE is set to 0V, and the control signals PU and PD are VDD and 0V, respectively. The level shifter  351  sets the gate of the transistor MN 2  to VDD. When receiving input signals of the VDDH level, the transistor MP 1  is switched on, which sets V CTRL  to the VDDH level so that a leakage path to the VDD line through the transistor MP 0  during the receiving mode may be prevented. When receiving input signals of the 0V level, due to a significant gate-to-source voltage, the transistor MP 2  is switched on, which sets V CTRL  to the VDD level during the receiving mode. During the receiving mode, either receiving input signals of the VDDH level or the 0V level, the transistor MP 0  is maintained at an off state. 
         [0033]    When the buffer circuit  30  operates in the transmitting mode, the OE signal is set to VDD. Both of the control signals PU and PD are set to VDD when transmitting output signals of the 0V level and set to 0V when transmitting output signals of the VDD level. The gate voltage of the transistor MN 2  is pulled up to the VDDH level by the level shifter  351 . The transistor MN 2  is switched on, which sets V CTRL  to the VDD level. The transistor MP 0  is maintained at the off state by the V CTRL  of the VDD level during the transmitting mode. As a result, the transistor MP 0  is turned off in both of the steady-states, i.e., the receiving mode and transmitting mode, and does not adversely affect the correct operations. In the steady states, the gate-oxide degradation and hot-carrier degradation are prevented in the buffer circuit  30 . 
         [0034]    During a transition from the state of receiving VDDH input signals to the state of transmitting 0-V output signals, the gate terminal of the transistor MN 1  is maintained at 0V while the PD signal is changing from 0V to VDD by the pre-driver  31 . In the meanwhile, the V CTRL  is set to the VDD level as the transistor MN 2  is switched on in response to the OE signal. Subsequently, the transistor MP 0  is turned on due to a significant gate-to-source voltage, and discharges the initial voltage of VDDH at the I/O pad  34 . After hundreds of picoseconds, for example, the voltage at I/O pad  34  is pulled down to approximately the VDD level, and the gate voltage of the transistor MN 1  increases to the VDD level after a delay induced by the inverter chain  371 . Therefore, the drain-to-source voltage of the transistor MN 0  is kept within the maximum normal operating voltage (V dd,nom ) range during the transition, resulting in no hot-carrier degradation. The V dd,nom  equals to approximately VDDH minus VDD. In the present example, the V dd,nom  is approximately 1.8V, given the VDD and VDDH being 1.5V and 3.3V, respectively. 
         [0035]      FIG. 4  is a schematic circuit diagram of a buffer circuit  40  consistent with an example of the present invention. Referring to  FIG. 4 , the buffer circuit  40  includes a pre-driver  41 , an input circuit  43 , an I/O pad  44 , a first hot-carrier-prevented (HCP) circuit  45 - 1 , a second HCP circuit  45 - 2  and a third HCP circuit  45 - 3 . Each of the HCP circuits  45 - 1 ,  45 - 2  and  45 - 3  includes a tracking circuit and a transistor controlled by the tracking circuit, which are similar in function to the tracking circuit  45  and the PMOS transistor MP 0  illustrated in  FIG. 3B . In one example according to the present invention, the buffer circuit  40  further includes a delay cell  47 , which includes an inverter chain  471  connected between the output enable signal OE and the pre-driver  41 . An output of the inverter chain  471  is connected to the gate of a transistor MN 4 . The delay cell  47  may further include a capacitor  472  connected between the output of the inverter chain  471  and the pre-driver  41  to provide a desirable delay time. 
         [0036]    During a transition from receiving VDDH input signals to transmitting 0-V output signals, the transistor MN 0  may risk the hot-carrier degradation due to a voltage V A  (which equals VDDH at the beginning of the transition) at the drain of the transistor MN 0 . With the first HCP circuit  45 - 1 , the voltage V A  is pulled down to the VDD level so that the transistor MN 0  is protected from the hot-carrier degradation. 
         [0037]    Meanwhile, at the beginning of the transition, the transistor MN 0  is weakly turned on so that V B  is approximately VDD. The transistor MP 5  is turned on because its gate and source are biased at V B  (VDD) and V A  (VDDH), respectively, which pulls V C  to V A  (VDDH). Similarly, during the transition from receiving VDDH input signals to transmitting 0-V output signals, the transistor MN 3  may risk the hot-carrier degradation due to the voltage V C  (VDDH) at the drain of the transistor MN 3 . With the second HCP circuit  45 - 2 , the voltage V C  is pulled down to VDD so that the transistor MN 3  is protected from the hot-carrier degradation. 
         [0038]    Furthermore, at the beginning of the transition, the transistor MP 3  is turned on because its gate and source are biased at VDD and V A  (VDDH), respectively, which pulls V D  to V A  (VDDH). During a transition from receiving VDDH input signals to transmitting VDD output signals, the control signal PU is set to 0V such that the transistors MN 2  and MP 2  may risk the hot-carrier degradation. With the third HCP circuit  45 - 3 , the voltage V D  is pulled down to VDD so that the transistors MN 2  and MP 2  are protected from the hot-carrier degradation. 
         [0039]      FIGS. 5A to 5C  are plots illustrating simulation results of the buffer circuit  40  illustrated in  FIG. 4 . The buffer circuit  40  meets the PCI-X 2.0 applications in a given 0.18-μm CMOS process, and transmits 0V-to-1.5V output signals and receives 0V-to-3.3V input signals. Furthermore, the buffer circuit  40  has an operating speed up to 266 mega Hertz (MHz). The hot-carrier effect is verified by Simulated Program with Integrated Circuits Emphasis (SPICE) simulation in a 0.18-μm CMOS process. 
         [0040]    Referring to  FIG. 5A , the drain-to-source voltage of the transistor MN 0  during the transition from receiving 3.3-V input signals to transmitting 0-V output signals is represented by a curve  52  illustrated in dotted lines. The peak of drain-to-source voltage of the transistor MN 0  is only approximately 1.8V, which is remarkably lower than that (2.8V) of a conventional buffer circuit represented by a curve  51 . Furthermore, the curve  52  is shifted relative to the curve  51  in the time axis due to a function of the delay cell  47 . 
         [0041]    Referring to  FIG. 5B , the drain-to-source voltage of the transistor MN 3  during the transition from receiving 3.3-V input signals to transmitting 0-V output signals is represented by a curve  54  illustrated in dotted lines. The peak of drain-to-source voltage of the transistor MN 3  is only approximately 1.7V, which is remarkably lower than that (2.7V) of a conventional buffer circuit represented by a curve  53 . 
         [0042]    Referring to  FIG. 5C , the drain-to-source voltage of the transistor MN 2  (or MP 2 ) during the transient from receiving 3.3-V input signal to transmitting 1.5-V output signal is represented by a curve  56  illustrated in dotted lines. The peak of drain-to-source voltage of the transistor MN 2  is only approximately 1.6V, which is remarkably lower than that (2.8V) of a conventional buffer circuit represented by a curve  55 . Therefore, in view of the simulation results illustrated in  FIGS. 5A to 5C , the potential hot-carrier effect on the transistors MN 0 , MN 3 , MN 2  and MP 2  has been suppressed by the HPC circuits  45 - 1 ,  45 - 2  and  45 - 3 . 
         [0043]    It will be appreciated by those skilled in the art that changes could be made to the examples described above without departing from the broad inventive concept thereof It is understood, therefore, that this invention is not limited to the particular examples disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 
         [0044]    Further, in describing representative examples of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.