Patent Publication Number: US-10784132-B2

Title: Method and apparatus for de-chucking a workpiece using a swing voltage sequence

Description:
This application is a Division of U.S. patent application Ser. No. 15/628,396 filed on Jun. 20, 2017, the entire contents of which are hereby incorporated by reference herein. 
    
    
     FIELD 
     The present description relates to microelectronics manufacturing and in particular to releasing a processed workpiece from an electrostatic chuck. 
     BACKGROUND 
     In semiconductor processing, a silicon wafer or other type of flat workpiece is exposed to sequence of processes to form layers and pattern of conductive and dielectric materials. These materials make up the transistors, connections, and other components of the eventual semiconductor die. The processes are performed in several different processing chambers. A chuck, carrier, pedestal, or another type of holder is used to hold the wafer as it is moved between chambers. The wafer may be carried by one carrier and then transferred to another carrier as it moves from one process to another. 
     An electrostatic chuck (ESC) is widely used as a holder to clamp on to a silicon or similar type of dielectric wafer in semiconductor plasma process chambers. The ESC uses an electrostatic force to grip the wafer during plasma and during other types of processes. At the end of a process or when the wafer is to be transferred to a different carrier, the electrostatic charge is discharged so that the wafer can be released from the ESC. 
     A two-step voltage sequence is sometimes used to de-chuck a wafer. The voltage sequence is to eliminate the electrostatic charge of the chuck that is applied to the wafer. The voltage sequence may be combined with a gentle helium gas pressure against the back side of the wafer to push it away from the chuck. 
     SUMMARY 
     A method and apparatus for de-chucking a workpiece is described that uses a swing voltage sequence. One example pertains to a method that includes applying a mechanical force from an electrostatic chuck against the back side of a workpiece that is electrostatically clamped to the chuck, applying a sequence of voltage pulses with a same polarity to the electrodes, each pulse of the sequence having a lower voltage than the preceding pulse, and determining whether the workpiece is released from the chuck after the sequence of additional voltage pulses and if the workpiece is not released then repeating applying the sequence of voltage pulses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which: 
         FIG. 1  is diagram of a multiple step voltage pulse swing according to an embodiment; 
         FIG. 2  is a process flow diagram of de-chucking a wafer according to an embodiment of the invention; 
         FIG. 3  is an alternative process flow diagram of de-chucking a wafer according to an embodiment of the invention; 
         FIG. 4  is an isometric top view of an electrostatic chuck in accordance with an embodiment of the invention; 
         FIG. 5  is a diagram of de-chucking system for an electrostatic chuck in accordance with an embodiment of the invention; and 
         FIG. 6  is a schematic of a plasma etch system including a chuck assembly in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A technique and apparatus is described herein to effectively de-chuck wafer using a multi-step pulsing swing voltage assisted by a back side helium gas pressure. Active control is used to control the steps and voltage applied to electrodes of the electrostatic chuck. 
     In some cases a two-step de-chuck sequence is not sufficient to release the wafer from the chuck. Electrostatic force generated by residual charge may result in wafer breakage. A helium leak is used as a trigger point indicating if electrical charge has been released from the wafer and the chuck electrodes. Depending on helium to gently press the wafer away from the chuck can result in a long de-chuck time or a wide variations in de-chuck times. If the de-chuck process takes too long, then a chamber control system may indicate a chamber fault due to the de-chuck sequence passing a timer setting for the chamber. 
     The de-chuck time varies with different structures that have been formed on the wafer, wafer front and backside coating, and chuck surface condition. While the static charge of the chuck may be easily dissipated, structures on the wafer may retain some residual charge. This charge then re-establishes a static charge between the wafer the chuck. Feature size continues to decrease and the transistor structures become increasingly complicated. In addition, transistors are often formed vertically instead of laterally. This increases the ability of the wafer to store and sustain an electrostatic charge. One such example is VNAND (Vertical Not AND) memory structures that use multiple dielectric layers deposited on the wafer front and back surfaces. Very high aspect ratio holes are used to make contacts or deep trenches for laying the infrastructure for electrical pathways. Such wafers may take a longer time, i.e. 30 minutes, to process the wafers. The various dielectric coating thickness, material electrical properties, long process times, and ESC surface conditions may cause complex electrical charge distributions at the interface between the wafer and the ESC ceramic layer during wafer processing with electrostatic chucking. All of these factors can require more time to fully release an electrostatic charge from a wafer during a de-chucking process. 
     As described herein, a wafer or other workpiece can be released or de-chucked from an electrostatic chuck by first supplying helium to the backside of the wafer to generate a uniform pushing force on the backside of the wafer. This accelerates the electrical charge applied to the wafer and releases from the wafer. A dechuck voltage sequence may also be applied at the same time that the helium pressure is applied to the wafer. 
     Shortly thereafter and while the back side gas is still being applied, a multi-step pulse swings through a de-chuck sequence. The voltage is damped with each step. Such a multi-step pulse swing is shown in  FIG. 1 . In  FIG. 1 , voltage amplitude is represented on the vertical axis and time is represented on the horizontal axis. The sequence starts with a negative pulse  4 . The original chucking voltage (not shown) was positive in this example, so this is followed by a negative voltage for dechucking. If the original chucking voltage had been negative then the de-chucking pulses would be positive and all of the polarities would be reversed as compared to those of  FIG. 1 . The first negative pulse  4  is followed by a sequence of negative pulses  6 ,  8 ,  10 . After each negative pulse, the voltage returns to a low positive voltage until the end of the sequence at  12  at which time either a low positive voltage is maintained, or the power is removed and the voltage is allowed to float. The indicated voltage is a voltage applied to the electrodes of the electrostatic chuck. 
     The initial positive chucking voltage is subjected to charged plasmas, ions, and other electrical sources. The first high negative voltage pulse  4  quickly removes charges built on the wafer above the dielectric coating of the chuck that continue to accumulate negative charge. The negative pulse generates a reverse coulomb force to balance residual positive electrostatic force, and give the wafer an impulse pushup. The second low positive voltage between the first 4 and second 6 negative pulses releases the negative charge built during the first negative pulse. The subsequent negative voltage pulses  6 ,  8  further release the residual charges on the interface between the wafer and the ESC. 
     The negative voltage is damped. In the illustrated example, there are four negative pulses and each pulse has half the amplitude of the one before it. The number of pulses and the amount of damping may be adapted to suit different wafers. The damping or reduction in pulse amplitudes also aids to avoid wafer pop-up and large wafer charge shifts. While the illustrated multi-step sequence uses square waves this is chosen to clearly illustrate the principles of the waveform. Other wave forms may alternatively be used. As examples a damped sine wave, a pulse width modulated voltage and other waveforms may be used. 
     During the application of the multi-step voltage pulse, the helium leak rate may be actively monitored. As helium is pumped against the back side of the wafer, it will be deflected radially toward the edges of the wafer and of the chuck. The rate at which the helium flows between the wafer and the chuck is measured by a helium controller, which measures the flow rate at a constant pressure, and in other ways. 
     The clamp force between the wafer and the chuck may be measured by this leak or flow rate. In some examples, if the leak rate is greater than some threshold, then a controller may determine that the wafer is no longer being held in place. The voltage may be released and lift pins, a manipulator arm or both may be applied to remove the wafer from the chuck. If the helium leak rate is below the threshold over a specified time, then the system may trigger another pulse swing de-chuck sequence. The multi-step pulse sequence may be repeated until the helium leak rate reaches the threshold or until the system times out and an alert, or fault can be signaled. 
       FIG. 2  is a process flow diagram for de-chucking a workpiece. The de-chucking process starts as a post process  20  to a processing operation that has been applied the wafer. While the processing operations are described herein as plasma processes applied to a chucked silicon wafer in a vacuum chamber, the principles herein apply also to other type of processes and workpieces. With the workpiece finished, mechanical force is applied to the back side of the wafer at  22 . This is gentle force that is not sufficient to release the wafer from the chuck without also applying voltage to the electrodes of the chuck. One type of gentle mechanical force is a gas pressure against the back side of the wafer. Many electrostatic chucks include a helium supply system to operate as a cooling gas against the back side of the wafer during plasma processing. This same cooling gas conduit may be used to press against the back side of the wafer. Many electrostatic chucks also include lift pins to lift the wafer off the top of the chuck. These may alternatively or additionally be used. 
     At  24 , a pulsing swing voltage is applied to the electrodes of the chuck. These electrodes may be the same electrodes that were used to initially apply an electrostatic force to grip or clamp the chuck before the processing. The pulsing swing voltage may be the same as that of  FIG. 1  or may take other forms as described above. The mechanical pressure  22  continues as the voltages are applied. 
     At  26  there is a timer. The timer is longer than the swing voltage time and expires after the voltage is relaxed. On the expiration of the timer, the pulsing sequence may be repeated at  24 . First the number of pulse sequences is counted as each pulse sequence is applied. Before each repeat this number is compared to a maximum threshold at  23 . Before the maximum number is reached then the voltage sequence is repeated. After the maximum number of sequences is reached then the de-chucking sequence is stopped at  27 . There may be a fault alarm to indicate that the wafer or process is not operating correctly since the wafer has not been released. The maximum threshold may vary with different wafer and process types and with different chucks. 
     At the same time, before the expiration of the timer, the mechanical pressure from a mechanical actuator such as lift pins or from a gas such as a helium leak rate is checked to determine whether the wafer is sufficiently released at  28 . As an example, the helium leak rate may be compared to a leak rate threshold T R . If the leak rate is below the threshold, then the wafer is not released and the process returns to the wait for the timer  26 . As mentioned above, this release state may be determined by testing the leak rate of an applied gas. If lift pins are used as the pressure, then the pressure against the lift pins may be measured. Other types of mechanical pressure  22  may be tested in other ways. If the wafer is sufficiently released, then at  30 , the wafer is ejected or removed from the chuck at  30 . 
       FIG. 3  is process flow diagram of an alternative process flow diagram for de-chucking a workpiece. In this example, the start  32  is also after the processing and the de-chucking is then a post process. Again the mechanical pressure is applied  34  to push gently against the back side of the wafer. Alternatively, the wafer may be gently pulled from the front side or the edges using a mechanical arm or actuator. The de-chucking voltage sequence is applied  36 , such as that of  FIG. 1  or various alternatives. At  38  before or after the sequence the helium leak rate or other mechanical parameter is measured. A timer  40  is then applied. 
     This timer  40 , like the timer of  FIG. 2  measures the elapsed time from starting the voltage sequence against a threshold. A second decision  41  tests the change of the helium gas leak rate at a specified time. This may be performed in a variety of different ways. In one example, the rate of change in the helium leak rate is compared to a threshold T C . If the change rate is less than the threshold, then the chuck clamping force is not being sufficiently affected and another voltage sequence is necessary to move the reduce the electrostatic force. The change in the leak rate gives an indication as to whether the wafer is being released. If the leak rate is not changing, then the wafer is not moving away from the chuck. A similar approach may be used for other types of mechanical force that is being applied to the chucked wafer. 
     If either condition, timer expiration or gas leak rate of change, is satisfied, then a yes condition is declared and the multiple step voltage pulse sequence is repeated at  36 . Otherwise, a no is declared and the helium leak rate is compared to a leak threshold at  42 . As in  FIG. 2 , this may be done by comparing the leak rate to T R . 
     This second helium leak rate is not a rate of change test as with the timer at  41 , but a current rate test. If the helium is leaking fast enough over leak threshold setting T R , then after a wait, the wafer is removed at  44 . The wait is used to allow the wafer to continue to work its way free from the chuck before applying a physical force to remove it. If no on condition  42 , the sequence moves back to step  38 . 
     If the timeout has occurred at  40  and the rate of change is small at  41 , then before the next voltage sequence, the number of sequences already used is compared to the maximum allowed. If the maximum number of sequences has been reached, then the de-chuck sequence is stopped at  48 . The system can send an alarm or declare a fault or alert. 
     The  FIG. 3  process may be understood in terms of several parameters that may be modified to suit different chamber conditions and to suit different wafer types. First there are the various voltages of the multiple step voltage sequence. These may be designated as V 1 , de-chuck voltage  1 , V 2 , de-chuck voltage  2 , etc. There are also timers for the processes, such as T 1 , a first de-chuck used to finish the multiple step voltage sequence and T 2 , a duration measured by the timer  40 . 
     There may be a leak rate threshold, T R , used to determine whether the leak rate is sufficient at  42 . There may be a leak rate of change threshold T C  used by the timer  40  to see if the wafer is starting to come free. A parameter Ns, a de-chuck swing cycle number, may be used to determine how many multiple step voltage sequences have been applied. This may be compared to a threshold N, a maximum number of de-chuck sequences that are permitted before a fault is applied. Td may be used to designate the delay time after satisfying the He leak threshold at  42  before the wafer is removed at  44 . 
     Considering these parameters more closely in  FIG. 3 , at  36 , Ns is initialized at start and incremented for each repetition from the timer. The multiple step voltage sequence  36  uses parameters V 1 , V 2 , V 3 , etc. and T 1 , etc. to drive the voltage sequence. The timer  40  uses the timeout timer T 2  and the gas leak rate of change T C  thresholds to determine a yes or a no. The gas leak rate threshold T R  is applied to determine whether the wafer is released, and if so then a timer Td is used to determine when to remove the wafer. 
     The parameter Ns introduces another criterion. Before the return loop is applied from  40  to  36 , the sequence count Ns is checked. If a sequence count threshold is exceeded, then instead of repeating the multiple step voltage sequence, an alert is triggered. An operator is alerted to determine if there is a fault in the process, the chuck, or the wafer. Alternatively, the wafer may be flagged as defective. In either event, the wafer must be removed from the chuck so that the chuck may be reused. 
       FIG. 4  is an isometric view of an assembled ESC suitable for use with the processes described above. A support shaft  212  supports a base plate  210  through a thermal isolator  216 . A cooling plate  208  and a heater plate  206  are carried by the base plate. A top heater plate  206  carries a puck  205  on the top surface of the heater plate. A workpiece  204 , such as a silicon substrate, glass sheet, or other material (not shown) is in turn carried above the puck and may be attached electrostatically. 
     The primary support shaft  212  supports the base plate  210  with an isolated thermal break  216  between the support shaft and the base plate. The shaft is hollow inside and includes conduits for conductors, gases and other materials that are supplied to the top of the chuck. The base plate supports the cooling plate  208 . The cooling plate is typically machined from aluminum and then covered with elastomer caps for each of the cooling channels. 
     The cooling plate absorbs heat from the workpiece through the dielectric puck  205  and the top plate  206 . It also absorbs heat from the top plate heaters. The top surface  205  of the top plate  206  is bonded to the dielectric puck  205  with a high temperature adhesive, such as silicone. The puck is typically ceramic but may alternatively be made with other materials. Electrodes (not shown) are embedded within the puck to generate an electrostatic field with which to grip a workpiece, such as a silicon substrate. 
     The base plate  210  provides a structural reinforcement to the cooling plate  208 . The base plate may be formed from a rigid material that has poor thermal conductivity or a lower thermal conductivity compared to the cooling plate. This prevents heat flow between cooling channels through the base plate. The base plate may be formed from polystyrene, titanium, alumina, ceramic, stainless steel, nickel, and similar materials. It may be formed of a single piece or several parts brazed together. The base plate may be bolted, screwed or riveted to the cooling plate, depending on the particular implementation. 
     The base plate  210  is carried on a shaft  212 . The shaft is hollow inside and includes conduits for conductors, gases and other materials that are supplied to the top of the chuck. An isolator  216  is placed between the metal shaft and the metal base plate  210  to reduce the conduction of heat between the shaft and the base plate. This keeps the shaft cooler and also shields heat from any handling mechanism that may be attached to the shaft. 
     While the present description is presented in the context of an electrostatic chuck such as the one shown in  FIG. 4 , the approaches and techniques may be applied to other types of electrostatic carrier, clamps, and pedestals. In some embodiments, the chuck does not include the heaters, cooling plates and other conduits but is used as a carrier for thinned wafers. The carrier is then held by a vacuum, electrostatic or other type of chuck. The de-chucking approach described herein may be used to remove a wafer from the carrier and also to remove the carrier from a chuck. 
       FIG. 5  is a diagram of a de-chucking system using a cooling gas supply and a controlled voltage source with an ESC. An ESC  240  acts as a support for a workpiece  242  and may be within a processing chamber or removed from the processing chamber with the workpiece  242  attached to its top plate. The workpiece may be a silicon wafer, or another type of workpiece such as gallium arsenide, lithium niobate, ceramic and other materials. 
     The chuck  240  may have a construction similar to that of  FIG. 4  and may include heating, cooling, and other features not shown in this diagram. The chuck includes electrodes  244  which may be in the form of a dipolar hemisphere or in any of a variety of other configurations that are connected through electrodes  243  to voltage source or power supply  246 . The power supply is regulated or commanded by a controller  248  to generate the various multiple step pulses as described herein. 
     The chuck also has one or more cooling gas outlets  250  at the top of the puck to direct air against the back side of the workpiece  242 . The gas outlets may be in any number and configuration depending on the design of the particular chuck. The chuck includes gas inlet fittings to allow a supply tube  252  to be coupled to a cooling gas source  258 , such as a Helium source. The He source provides the cooling gas under pressure to a control valve  256 . The gas flows from the source to the control valve and then through a flow rate detector  254  to the chuck. The flow rate detector measures the flow rate of the gas which is directly related to the gas leakage rate. It may also measure the gas pressure. These results are provided to the connected controller  248  which may then control the valve  256  and operate the processes of  FIGS. 2 and 3 . Alternatively the detector  243  may be a part of the valve or the gas source. 
     The chuck also has lift pins  260  that may be connected through electrical connectors  243  to the controller  248  to operate in pushing the workpiece off of the chuck. These components operate together to accomplish the functions described above to de-chuck the workpiece. 
     As shown in the figure, the de-chucking system operates on an electrostatic chuck that has electrodes that are used to apply the electrostatic clamping force to the workpiece. There is a mechanical actuator coupled to the chuck to apply a mechanical force against the back side of the workpiece that is electrostatically clamped to the chuck. This may be a gas or a physical structure like a lift pin a robot arm or some other actuator on the back side, the edges or even a vacuum chuck on the front side. There is the power supply coupled to the chuck to apply voltages to the electrodes of the chuck and then a controller to drive the power supply. The power supply applies a sequence of voltage pulses with a same polarity to the electrodes. Each pulse of the sequence has a lower voltage than the preceding pulse. The controller determines whether the workpiece is released from the chuck after the sequence of additional voltage pulses and if the workpiece is not released then the controller drives the power supply to repeat applying the sequence of voltage pulses. 
       FIG. 6  is a schematic of a plasma etch system  100  including a chuck assembly  142  in accordance with an embodiment of the present invention. The plasma etch system  100  may be any type of high performance etch chamber known in the art, such as, but not limited to, Enabler®, DPS II®, AdvantEdge™ G3, EMAX®, Axiom®, or Mesa™ chambers, all of which are manufactured by Applied Materials of California, USA. Other commercially available etch chambers may similarly utilize the chuck assemblies described herein. While the exemplary embodiments are described in the context of the plasma etch system  100 , the chuck assembly described herein is also adaptable to other processing systems used to perform any plasma fabrication process (e.g., plasma deposition systems, etc.) 
     Referring to  FIG. 6 , the plasma etch system  100  includes a grounded chamber  105 . Process gases are supplied from gas source(s)  129  connected to the chamber through a mass flow controller  149  to the interior of the chamber  105 . Chamber  105  is evacuated via an exhaust valve  151  connected to a high capacity vacuum pump stack  155 . When plasma power is applied to the chamber  105 , a plasma is formed in a processing region over a workpiece  110 . A plasma bias power  125  is coupled into the chuck assembly  142  to energize the plasma. The plasma bias power  125  typically has a low frequency between about 2 MHz to 60 MHz, and may be, for example, in the 13.56 MHz band. In an example embodiment, the plasma etch system  100  includes a second plasma bias power  126  operating at about the 2 MHz band which is connected to an RF (Radio Frequency) match  127 . The plasma bias power  125  is also coupled to the RF match and also coupled to a lower electrode via a power conduit to supply the drive current  128 . A plasma source power  130  is coupled through another match (not shown) to a plasma generating element  135  to provide high frequency source power to inductively or capacitively energize the plasma. The plasma source power  130  may have a higher frequency than the plasma bias power  125 , such as between 100 and 180 MHz, and may, for example, be in the 162 MHz band. 
     A workpiece  110  is loaded through an opening  115  and clamped to the chuck assembly  142  inside the chamber. The workpiece  110 , such as a semiconductor wafer, may be any wafer, substrate, or other workpiece employed in the semi-conductor processing art and the present invention is not limited in this respect. The workpiece  110  is disposed on a top surface of a dielectric layer or puck of the chuck assembly that is disposed over a cooling base assembly  144  of the chuck assembly. A clamp electrode (not shown) is embedded in the dielectric layer. In particular embodiments, the chuck assembly  142  includes electrical heaters (not shown). The heaters may be independently controllable to the same or to different temperature set points. 
     A system controller  170  is coupled to a variety of different systems to control a fabrication process in the chamber. The controller  170  may include a temperature controller  175  to execute temperature control algorithms (e.g., temperature feedback control) and may be either software or hardware or a combination of both software and hardware. The temperature controller receives temperature information from a thermal sensor  143  in the chuck and then adjusts the heaters and heat exchangers accordingly. While only one thermal sensor is shown, there may be many more in many different locations, depending on the particular implementation. The system controller  170  also includes a central processing unit  172 , memory  173  and input/output interface  174 . The temperature controller  175  is to output control signals or drive current  128  to the heaters affecting the rate of heating and therefore the rate of heat transfer between each heater zone of the chuck assembly  142  and the workpiece 
     In embodiments, in addition to the heaters, there may be one or more coolant temperature zones. The coolant zones have heat transfer fluid loops with flow control that is controlled based on a temperature feedback loop. In the example embodiment, the temperature controller  175  is coupled through a control line  176  to a heat exchanger (HTX)/chiller  177  depending on the particular implementation. The control line may be used to allow the temperature controller to set a temperature, flow rate, and other parameters of the heat exchanger. The flow rate of the heat transfer fluid or coolant through conduits in the chuck assembly  142  may alternatively or additionally be controlled by the heat exchanger. 
     One or more valves  185  (or other flow control devices) between the heat exchanger/chiller  177  and fluid conduits in the chuck assembly  142  may be controlled by the temperature controller  175  to independently control a rate of flow of the heat transfer fluid. The temperature controller may also control the temperature set point used by the heat exchanger to cool the heat transfer fluid. 
     The heat transfer fluid may be a liquid, such as, but not limited to deionized water/ethylene glycol, a fluorinated coolant such as Fluorinert® from 3M or Galden® from Solvay Solexis, Inc. or any other suitable dielectric fluids such as those containing perfluorinated inert polyethers. While the present description describes a vacuum chuck in the context of a plasma processing chamber, the principle, structures, and techniques described herein may be used with a variety of different workpiece supports, in a variety of different chambers and for a variety of different processes. 
     The processing system  100  also includes cooling gas source  181  that is coupled through a control valve  182  to the chuck assembly  142 . The cooling gas source may be a source of He or any other suitable gas to conduct heat between the puck and the workpiece to conduct heat away from the workpiece. The cooling gas source may also be coupled to other components for other purposes. As described, the gas is supplied to the chuck and flowed against the back side of the workpiece. A separate source of reactive gases  129  is provided to support the fabrication processes being applied to the workpiece  110 . The cooling gas source may be used to support other processes, depending on the particular implementation. 
     In operation, a workpiece is moved through the opening of the chamber and attached to the puck of the carrier for fabrication processes. Any of a variety of different fabrication process may be applied to the workpiece while it is in the processing chamber and attached to the carrier. During the process and optionally before the process, the dry gas is supplied under pressure to the dry gas inlet of the base plate. The pressure pushes the dry gas into the space between the base plate and the cooling plate. The gas flow drives the ambient air from between the base plate and the cooling plate. 
     As used in the description of the invention and the appended claims, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. 
     The terms “coupled” and “connected,” along with their derivatives, may be used herein to describe functional or structural relationships between components. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical, optical, or electrical contact with each other. “Coupled” my be used to indicate that two or more elements are in either direct or indirect (with other intervening elements between them) physical, optical, or electrical contact with each other, and/or that the two or more elements co-operate or interact with each other (e.g., as in a cause an effect relationship). 
     The terms “over,” “under,” “between,” and “on” as used herein refer to a relative position of one component or material layer with respect to other components or layers where such physical relationships are noteworthy. For example in the context of material layers, one layer disposed over or under another layer may be directly in contact with the other layer or may have one or more intervening layers. Moreover, one layer disposed between two layers may be directly in contact with the two layers or may have one or more intervening layers. In contrast, a first layer “on” a second layer is in direct contact with that second layer. Similar distinctions are to be made in the context of component assemblies. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, while flow diagrams in the figures show a particular order of operations performed by certain embodiments of the invention, it should be understood that such order is not required (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.). Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.