Patent Publication Number: US-11658480-B2

Title: Ultra-low leakage electrostatic discharge device with controllable trigger voltage

Description:
BACKGROUND 
     The present disclosure relates to integrated circuits, and more specifically, to an electrostatic discharge (ESD) device with ultra-low current leakage and a controllable trigger voltage. 
     An integrated circuit (IC) may be exposed to random ESD events that can direct potentially large and damaging ESD currents to sensitive electronic components of the IC. An ESD event may occur, for example, during post-manufacture chip handling or after chip installation on a circuit board or other carrier. During an ESD event, an electrostatic charge is rapidly transferred between two objects at different electric potentials when the objects come into direct contact with each other. 
     To protect against ESD events, an IC may be provided with one or more ESD devices. Such ESD devices are generally designed to remain quiescent during normal operation of the IC and to turn on in response to an ESD event. When provided on an IC, designers must take into consideration the operating voltage, leakage current, breakdown constraints, and area footprint of the ESD devices. 
     A typical ESD device may have a trigger voltage (Vt) that is higher than the operating voltage of the IC, with the ESD device remaining in a quiescent state until an applied voltage due to an ESD event reaches or exceeds Vt. When an ESD event occurs, the ESD device enters a low-impedance state that conducts ESD current to ground and away from the sensitive electronic components of the IC. The ESD device may remain (e.g., clamped) in its low-impedance state until the ESD current is drained and the ESD voltage is discharged to an acceptable level. 
     Designers are often tasked with reducing the power consumption of ICs (e.g., by reducing leakage currents of ESD devices and other components). The current leakage of ESD devices is an important factor for the energy consumption of a circuit application. This is especially true for mobile electronic devices such as smart phones, tablets, wearables, or medical devices as the off-state current leakage may influence battery life and therefore operation time. Although ultra-low-leakage (ULL) ESD devices have been developed, such ULL ESD devices generally require a trigger voltage that is much higher than the operating voltages of other circuits/devices on the IC. Although the trigger voltage may be adjusted (e.g., using additional implants), this generally leads to higher costs and higher leakage currents. 
     Examples of ESD devices with reduced leakage currents include gated silicon controlled rectifiers (SCRs), diode strings, and SCRs with a feedback loop. However, such ESD devices may have a large area footprint and increased cost. Further, such ESD devices are incapable of providing low leakage currents together with a controllable trigger voltage. 
     SUMMARY 
     A first aspect of the disclosure is directed to an electrostatic discharge (ESD) device, including: an input pad; an underlapped field effect transistor (UL-FET) with a trigger voltage Vt, including: an underlapped drain region coupled to the input pad; a source region coupled to ground; and a gate structure coupled to the input pad; and a blocking layer separating the underlapped drain region from the gate structure of the UL-FET by an underlap distance. 
     A second aspect of the disclosure is directed to a method for electrostatic discharge (ESD) protection, including: coupling an ESD device to a circuit to be protected, the ESD device including an underlapped field effect transistor (UL-FET) with an underlapped drain region; and controlling a trigger voltage of the UL-FET by at least one of: adjusting a gate voltage Vg applied to a gate structure of the UL-FET; adjusting a capacitance of a capacitor C coupled between the gate structure of the UL-FET and an input pad; and adjusting an underlap distance between the underlapped drain region of the UL-FET and the gate structure of the FET. 
     A third aspect of the disclosure is directed to a electrostatic discharge (ESD) device, including: an input pad; an underlapped n-type field effect transistor (UL-NFET) with a trigger voltage Vt, including: an underlapped n+-doped drain region formed in a p-well and coupled to the input pad; an n+-doped source region formed in the p-well and coupled to ground; and a gate structure coupled to the input pad; and a silicide blocking layer separating the n+-doped underlapped drain region from the gate structure of the UL-NFET by an underlap distance. 
     The foregoing and other features of the disclosure will be apparent from the following more particular description of embodiments of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of this disclosure will be described in detail, with reference to the following figures, wherein like designations denote like elements. 
         FIG.  1    depicts an ultra-low-leakage (ULL) electrostatic discharge (ESD) device with controllable trigger voltage according to embodiments. 
         FIG.  2    depicts an equivalent electrical circuit for the ULL ESD device of  FIG.  1    according to embodiments. 
         FIG.  3    illustrates the snapback current Id (log scale) versus drain voltage Vd of the ESD device of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =0. 
         FIG.  4    illustrates the snapback current Id (log scale) versus drain voltage Vd of the ESD device of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =50 nm. 
         FIG.  5    illustrates the snapback current Id (log scale) versus drain voltage Vd of the ESD device of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =100 nm. 
         FIG.  6    illustrates the snapback current Id (linear scale) versus drain voltage Vd of the ESD device of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =0. 
         FIG.  7    illustrates the snapback current Id (linear scale) versus drain voltage Vd of the ESD device of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =50 nm. 
         FIG.  8    illustrates the snapback current Id (linear scale) versus drain voltage Vd of the ESD device of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =100 nm. 
         FIG.  9    depicts a table showing the trigger voltage Vt of the ESD device of  FIG.  1    according to embodiments for different gate voltages Vg and underlap distances X 1 . 
         FIG.  10    depicts an ESD device and an npn device for providing a discharge path according to embodiments. 
         FIG.  11    depicts an ESD device and a silicon-controlled-rectifier (SCR) device for providing a discharge path according to embodiments. 
     
    
    
     It is noted that the drawings of the disclosure are not necessarily to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration specific exemplary embodiments in which the present teachings may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present teachings, and it is to be understood that other embodiments may be used and that changes may be made without departing from the scope of the present teachings. The following description is, therefore, merely illustrative. 
     An electrostatic discharge (ESD) device with ultra-low-leakage (ULL) current and a controllable trigger voltage according to embodiments is described herein. The ULL ESD device according to embodiments may include an underlapped field effect transistor (UL-FET). The gate voltage of the ULL ESD device may be dynamically controlled by the voltage applied to the gate of the UL-FET. The ULL ESD device may be tuned by adjusting a capacitance coupled to the gate of the UL-FET or varying the degree of underlap of the UL-FET. The ULL ESD device is compatible with existing masks and processes and may be provided without a significant cost increase or area penalty. 
       FIG.  1    depicts an ultra-low-leakage (ULL) electrostatic discharge (ESD) device  100  (hereafter ESD device  100 ) with controllable trigger voltage according to embodiments.  FIG.  2    depicts an equivalent electrical circuit for the ESD device  100  of  FIG.  1    according to embodiments. As shown in  FIGS.  1  and  2   , the ESD device  100  may include an underlapped n-type field effect transistor (UL-NFET)  102 , a capacitor  104  (e.g., a back end of the line (BEOL) capacitor), and an n-type switching FET (NFET)  106 . 
     The UL-NFET  102  may be formed in a p-type doped region  108  (e.g., hereafter p-well  108 ) and may include an n+-doped source region  110 , an underlapped n+-doped drain region  112 , and a gate structure  114 . By ‘underlapped region’ it is meant a diffusion region in which no part thereof is located directly beneath the gate structure  114 . As shown in  FIG.  1   , for example, the underlapped n+-doped drain region  112  is spaced away from the edge of the gate structure  114  and an underlying gate channel region  116  by an underlap distance X 1 . The various doped regions described herein may be formed in any known manner (e.g., via ion implantation of dopants). The various gate structures described herein may also be formed in any known manner. 
     A silicide blocking layer, including a source-side silicide blocking (SOP) region  118  and a drain-side silicide blocking (DOP) region  120 , may be formed over portions of the p-well  108 , n+-doped source region  110 , and underlapped n+-doped drain region  112  as shown in  FIG.  1   . The forming of the silicide blocking layer may include, for example, depositing an insulating layer and selectively etching the insulating layer. The silicide blocking layer may, for example, be formed of silicon oxide or silicon nitride. 
     According to embodiments, the underlapped n+-doped drain region  112  may be coupled to an input terminal or pad  122  (hereafter input pad  122 ). The gate structure  114  of the UL-NFET  102  may also be coupled to the input pad  122  through the capacitor  104 . The gate structure  114  of the UL-NFET  102  may further be latched to ground through the switching NFET  106 . In the equivalent circuit depicted of the ESD device  100  depicted in  FIG.  2   , the intrinsic gate-source capacitance of the UL-NFET  102  is represented by the capacitor Cgs. To this extent, the gate voltage Vg applied to the gate structure  114  of the UL-NFET  102  may be set by the voltage divider formed by the series connection of the capacitor  104  and the capacitor Cgs. According to other embodiments, the capacitor  104  may be replaced by the intrinsic gate-drain capacitance Cgd of the UL-NFET  102 . According to further embodiments, an additional capacitor may be coupled in parallel to the intrinsic gate-source capacitance Cgs of the UL-NFET  102  to better control the gate voltage Vg. 
     The switching NFET  106  may also be formed in the p-well  108 . The switching NFET  106  may include an n+-doped drain region  124 , the n+-doped source region  110 , and a gate structure  126 . According to embodiments, the n+-doped drain region  110  may be shared by the switching NFET  106  and the UL-NFET  102 . The gate structure  124  of the switching NFET  106  may be coupled to a supply voltage Vdd. 
     The switching NFET  106  may be used to control the operation of the UL-NFET  102 . When Vdd is high, the gate voltage Vg at the gate structure  114  of the UL-NFET  102  is latched to ground, deactivating the UL-NFET  102 . When Vdd is low, the switching NFET  106  is in floating state and the gate voltage Vg is dependent upon the voltage at the input pad  122  and the capacitance values of the capacitor  104  and the intrinsic gate-source capacitance Cgs of the UL-NFET  102 . 
     Referring again to  FIGS.  1  and  2   , during an ESD event  130  (represented by the lightning bolt at the input pad  122 ), a gate voltage Vg is applied to the gate structure  114  of the UL-NFET  102  of the ESD device  100 . As detailed above, the gate voltage Vg is dependent upon the voltage at the input pad  122  and the capacitance values of the capacitor  104  and the intrinsic gate-source capacitance Cgs of the UL-NFET  102 . The ESD device  100  is activated when the voltage between the underlapped n+-doped drain region  112  and the n+-doped drain region  110  is larger than the trigger voltage Vt of the UL-NFET  102 . The trigger voltage Vt is modulated by the gate voltage Vg 
     Many ESD devices are voltage clamping devices with high resistance up to a trigger (e.g., breakdown) voltage above which their resistance drops dramatically. They turn on at that point and begin to conduct current. For such ESD devices, their trigger voltage (turn on) is above the normal operating voltage of the protected circuit, but low enough that the clamping voltage is below a voltage that can damage the protected circuit. Voltage clamping devices are ideally switched off during normal operations (allowing a negligible current through) but begin conducting current once their threshold voltage has been reached. 
     Looking at the internal structure of a solid state ESD device, a free electron (hole) becomes mobile in the presence of an electric field and if the electric field is low, the electron moves about without incident. On the other hand, a strong electric field produced by a high voltage associated with an ESD event energizes a mobile electron to strike and free bound electrons. The free electrons propagate the same action which leads to an avalanche of electrons (breakdown). 
     Snapback ESD devices, such as the ESD device  100  according to embodiments, behave differently. A typical MOSFET has a parasitic bipolar junction transistor which has a source as its emitter and drain as its collector. As the avalanche current flows to the base of the parasitic bipolar junction transistor, the base current can trigger the ESD device to allow current flows between the collector and emitter. Once triggered, the strong electric field that caused the avalanche current is no longer necessary to sustain it. Conduction of large currents continues even at lower voltages. 
     At low voltages, a snapback ESD device also has high resistance until it turns on at a trigger voltage Vt greater than normal operating conditions of the protected circuit. After the snapback ESD device begins to conduct (now with a low resistance), the voltage drops below the trigger voltage Vt toward the normal operating voltage of the protected circuit. That is, the device could remain in the on state, conducting current even after the ESD event is over. 
     An ESD protection device should be selected such that the voltage at which it triggers is not too high that it can damage sensitive components of the circuit to be protected. Similarly, the lower holding voltage should not be lower than the normal operating voltage of the circuit so that the device switches off after the ESD event ends. 
       FIG.  3    illustrates the snapback current Id (log scale) versus drain voltage Vd of the ESD device  100  according to embodiments for different gate voltages Vg and an underlap distance X 1 =0.  FIG.  4    illustrates the snapback current Id (log scale) versus drain voltage Vd of the ESD device  100  according to embodiments for different gate voltages Vg and an underlap distance X 1 =50 nm.  FIG.  5    illustrates the snapback current Id (log scale) versus drain voltage Vd of the ESD device  100  according to embodiments for different gate voltages Vg and an underlap distance X 1 =100 nm. 
     In  FIGS.  3 - 5   , the snapback current Id for a gate voltage Vg=0V represents the leakage current of the ESD device  100 . Comparing  FIGS.  3  and  4   , for example, it can be seen that the leakage current (i.e., snapback current at Vg=0V) for an overlap distance X 1 =50 nm ( FIG.  4   ) is substantially smaller than the leakage current for an overlap distance X 1 =0 ( FIG.  3   ). Indeed, at a drain voltage Vd of 5 V and an underlap distance X 1 =0 ( FIG.  3   ), the leakage current is approximately 100 nA, while at a drain voltage Vd of 5 V and an underlap distance X 1 =50 nm ( FIG.  4   ), the leakage current is approximately 0.01 nA. To this extent, the leakage current is reduced by approximately three orders of magnitude (e.g., 1000×) when providing an underlap distance X 1 =50 nm compared to an underlap distance of X 1 =0. The use of an underlap distance X 1 =100 nm affords a similar reduction in leakage current when compared to an underlap distance X 1 =0. From this and other data presented herein, one can thus extrapolate that the leakage current is reduced for an underlapped distance X 1 &gt;0. 
       FIGS.  6 - 8    depict the same data as that shown in  FIGS.  3 - 5   , respectively, but using a different scale (i.e., linear vs. log) for the snapback current Id. In particular,  FIG.  6    illustrates the snapback current Id (linear scale) versus drain voltage Vd of the ESD device  100  of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =0.  FIG.  7    illustrates the snapback current Id (linear scale) versus drain voltage Vd of the ESD device  100  of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =50 nm.  FIG.  8    illustrates the snapback current Id (linear scale) versus drain voltage Vd of the ESD device  100  of  FIG.  1    according to embodiments for different gate voltages Vg and an underlap distance X 1 =100 nm. 
     Comparing  FIGS.  6 - 8   , it can be seen that the trigger voltage Vt required for turning on the ESD device  100  may be dynamically controlled in a number of ways. For example, the trigger voltage Vt may be controlled based on the overlap distance X 1  of the ESD device  100  and/or the gate voltage Vg applied to the gate structure  114  of the UL-NFET  102  of the ESD device  100 . For example,  FIG.  9    depicts a table (Table 1) showing the relationship between the trigger voltage Vt, underlap distance X 1 , and gate voltage Vg of the ESD device  100  of  FIG.  1   , based on the data provided in  FIGS.  6 - 8   . 
     From Table 1, it can be seen that in the case of no underlap (underlap distance X 1 =0), a gate voltage Vg of about 0.2 volts results in a trigger voltage Vt of about 6 V, while a gate voltage Vg of about 0.4 volts results in a trigger voltage Vt of about 4.2 V. A greater range of trigger voltages Vt are possible for an underlap distance X 1 =50 nm. For example, again from Table 1, it can be seen that in the case of an underlap distance X 1 =50 nm, a gate voltage Vg of about 0.2 volts results in a trigger voltage Vt of about 10.5 V, while a gate voltage Vg of about 0.8 volts results in a trigger voltage Vt of about 3.5 V. The gate voltage Vg, which may be set by the voltage divider formed by the series connection of the capacitor  104  and the intrinsic gate to source capacitance Cgs, can also be tuned by varying the capacitance of the capacitor  104  coupled to the gate structure  114  of the UL-NFET  102 . In summary, the trigger voltage Vt can be dynamically tuned over a wide range by varying at least one of: 
     1) The gate voltage Vg; 
     2) The capacitance of the capacitor  104 ; and 
     3) The underlap distance X 1 . 
     The ESD device  100  provides a trigger path (TP) for current in response to an ESD event  130  at the input pad  122 . As will be described in detail below with regard to  FIGS.  10  and  11   , current flowing along the trigger path in response to an ESD event  130  actuates a discharge path (DP) through a discharge path device that safely dissipates the ESD current to ground. 
       FIG.  10    depicts an ESD device  200  including an underlapped n-type field effect transistor (UL-NFET)  202  and an npn device  250  for providing a discharge path according to embodiments. The UL-NFET  202  may be formed in a p-type doped region  208  (e.g., hereafter p-well  208 ) and may include an n+-doped source region  210 , an underlapped n+-doped drain region  212 , and a gate structure  214 . The p-well  208  may be separated from a substrate  252  by an n-type doped region  254  (e.g., an n-well). Again, as described above, an ‘underlapped region’ it is meant a diffusion region in which no part thereof is located directly beneath the gate structure  214 . As shown in  FIG.  10   , for example, the underlapped n+-doped drain region  212  is spaced away from the edge of the gate structure  214  and an underlying gate channel region  216  by an underlap distance X 1 . The various doped regions described herein may be formed in any known manner (e.g., via ion implantation). The various gate structures described herein may also be formed in any known manner. 
     A silicide blocking layer, including a source-side silicide blocking (SOP) region  218  and a drain-side silicide blocking (DOP) region  220 , may be formed over the p-well  208  and underlapped n+-doped drain region  212 . The forming of the silicide blocking layer may include, for example, depositing an insulating layer and selectively etching the insulating layer. The silicide blocking layer may, for example, be formed of silicon oxide or silicon nitride. 
     According to embodiments, the underlapped n+-doped drain region  212  may be coupled to an input terminal or pad  222  (hereafter input pad  222 ). The gate structure  214  of the UL-NFET  202  may also be coupled to the input pad  222  through a capacitor  204 . The gate structure  214  and the n+-doped source region  210  of the UL-NFET  202  may be coupled to ground through resistors R 1  and R 2 , respectively. The n+-doped source region  210  may also be coupled to a p+-doped region  256  formed in p-well  208 , and may be isolated from the n+-doped source region  210  by an isolation region  258 , such as a shallow trench isolation region or a silicon blocking region. The isolation region(s) described herein may be formed in any known manner, for example, by etching a pattern of trenches in the p-well  208 , depositing one or more dielectric materials (such as silicon dioxide) to fill the trenches, and removing the excess dielectric using a technique such as chemical-mechanical planarization. Similar to the ESD device  100  depicted in  FIG.  1   , the gate voltage Vg applied to the gate structure  214  of the UL-NFET  202  may be set by a voltage divider formed by the series connection of the capacitor  204  and the intrinsic gate-source capacitance of the UL-NFET  202 . 
     The npn device  250  may be formed by the underlapped n+-doped drain region  212 , which is shared by the UL-NFET  202 , an n+-doped region  260  formed in the p-well  208 , and the p-well  208 . As shown in  FIG.  10   , the underlapped n+-doped drain region  212  is isolated from the n+-doped region  260  by an isolation region  258 . 
     The UL-NFET  202  may be activated in response to an ESD event  230  at the input pad  222 . For example, the UL-NFET  202  may be activated when the voltage between the underlapped n+-doped drain region  212  and the n+-doped source region  210  is larger than the trigger voltage Vt of the UL-NFET  202 . In such a case, current flows along a trigger path (TP) from the underlapped n+-doped drain region  212  to the n+-doped source region  210  and subsequently to ground through the resistor R 2 . The raises the bias of the p-well  208  via the p+-doped region  256 , activating the npn device  250 . When the npn device  250  activated, current flows along a discharge path (DP) from the underlapped n+-doped drain region  212  to ground through the n+-doped region  260 . 
       FIG.  11    depicts an ESD device  300  including an n-type field effect transistor (UL-NFET)  302  and a silicon-controlled-rectifier (SCR) device  350  for providing a discharge path according to embodiments. The UL-NFET  302  may be formed in a p-type doped substrate  308  and may include an n+-doped source region  310 , an underlapped n+-doped drain region  312 , and a gate structure  314 . Again, as described above, an ‘underlapped region’ it is meant a diffusion region in which no part thereof is located directly beneath the gate structure  314 . As shown in  FIG.  11   , for example, the underlapped n+-doped drain region  312  is spaced away from the edge of the gate structure  314  and an underlying gate channel region  316  by an underlap distance X 1 . The various doped regions described herein may be formed in any known manner (e.g., via ion implantation). The various gate structures described herein may also be formed in any known manner. 
     A silicide blocking layer, including a source-side silicide blocking (SOP) region  318  and a drain-side silicide blocking (DOP) region  320 , may be formed over the p-type doped substrate  308  and underlapped n+-doped drain region  312 . The forming of the silicide blocking layer may include, for example, depositing an insulating layer and selectively etching the insulating layer. The silicide blocking layer may, for example, be formed of silicon oxide or silicon nitride. 
     According to embodiments, the gate structure  314  of the UL-NFET  302  may be coupled to an input terminal or pad  322  (hereafter input pad  322 ). through a capacitor  304 . The gate structure  314  of the UL-NFET  302  may also be coupled to ground through a resistor R. The n+-doped source region  310  of the UL-NFET  302  may also be coupled to ground. A p+-doped substrate contact  356  may be formed in the p-type doped substrate  308  and may be isolated from the n+-doped source region  310  by an isolation region  358 , such as a shallow trench isolation region or a silicon blocking region. According to embodiments, the gate voltage Vg applied to the gate structure  314  of the UL-NFET  302  may be set by a voltage divider formed by the series connection of the capacitor  304  and the intrinsic gate-source capacitance Cgs of the UL-NFET  302 . 
     The SCR device  350  according to embodiments may include an n-type doped region (hereafter n-well  360 ) and a p-type doped region (hereafter p-well  362 ). Further, the SCR device  350  may include the underlapped n+-doped drain region  312  (which extends partially over the p-type doped substrate  308  and the n-well  360 ), a p+-type doped region  364  separated from the underlapped n+-doped drain region  312  by an isolation region  358  and coupled to the input pad  322 , an n+-type doped region  366  formed in the p-well  362  and coupled to ground, and a p+-type doped region  368  formed in the p-well  362  and coupled to ground. Isolation regions  358  may be formed between the p+-type doped region  364  and the n+-type doped region  366 , and between the n+-type doped region  366  and the p+-type doped region  368 . 
     The UL-NFET  302  may be activated in response to an ESD event  330  at the input pad  322 . When a positive ESD current is applied on the input pad  322 , the diode formed by the p+-type doped region  364  and the n-well  360  is forward biased and current flows into the nwell  360  which is connected to the underlapped n+-doped drain region  312  of the UL-NFET  302 . The UL-NFET  302  may then be activated when the voltage between the underlapped n+-doped drain region  312  and the n+-doped source region  310  is larger than the trigger voltage Vt of the UL-NFET  302 . In such a case, current flows along a trigger path (TP) from the p+-doped region  364  to the n+-doped source region  310 . This current path injects holes into the nwell  360  of the SCR which may trigger the SCR  350  and provide a discharge path to ground. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” “approximately,” and “substantially,” are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. “Approximately” as applied to a particular value of a range applies to both values, and unless otherwise dependent on the precision of the instrument measuring the value, may indicate +/−10% of the stated value(s). 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.